Biological Rhythms: Implications for the Worker (Part 7 of 19)

Chapter 5

Biological Rhythms andWork Schedules

Contents

WORK SCHEDULES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87METHODOLOGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88STRESSORS CAUSED BY WORK SCHEDULES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

Disruption of Circadian Rhythms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89Sleep Disruption and Fatigue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92Social and Domestic Disturbances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

CONSEQUENCES OF WORK-RELATED STRESSORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95Health . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

INTERVENTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102Work Schedules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102Sleeping and Napping Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107Medication and Drug Therapies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111Employee Education and Clinical Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111Fitness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

RESEARCH NEEDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112SUMMARY AND CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113CHAPTER 5 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

BoxesBox Page5-A. Night Shift Paralysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 935-B. The Grounding of the Exxon Valdez . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1035-C. The Philadelphia Police Story . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1045-D. Astronauts and Bright Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

FiguresFigure Page

5-1.5-2.5-3.5-4.5-5.5-6.5-7.5-8.5-9.

5-1o.5-11.

Table

Relationship of Body Temperature, Work, and Sleep on a Permanent Night Shift . . . . . . 90Relationship of Body Temperature, Work, and Sleep in a Rapily Rotating Schedule . . . 91Length of Sleep of Permanent and Rotating Shift Workers . . . . . . . . . . . . . . . . . . . . . . . . . . . 92Sleep Duration and Fatigue in Aircrews During a 4-Day Trip . . . . . . . . . . . . . . . . . . . . . . . . . 94Length of Sleep of Permanent Night Shift Workers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96Shifts Interfering With Eating Habits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .97prevalence of Disease in Shift Workers and Day Workers . . . . . . . . . . . . . . . . . . . . . . . . . . . 98Work Performance in Various Job Settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99Fatigue-Related Vehicular Accidents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102Measures of Performance on 8-Hour and 12-Hour Shifts . . . . . . . . . . . . . . . . . . . . . . . . . . . 106Effects of Bright Light on the Adjustment of One Worker to Shift Work . . . . . . . . . . . . 109

5-C-1. Original and Newly Instituted

TablePage

Schedules for the Philadelphia Police Study . . . . . . . . . 104

Chapter 5

Biological Rhythms and Work Schedules

As discussed in chapter 3, disruption of biologicalrhythms can result in physiological changes thatadversely affect both health and performance. In theworkplace, certain types of work schedules canrequire persons to work at inappropriate points in thecircadian cycle and cause disruptions of biologicalrhythms, notably circadian rhythms. However, inthese situations, the physiological changes causedby disruption of circadian rhythms often interactwith other stressors associated with work schedules(fatigue, sleep deprivation, and social or domesticstress) to compound the effects. This chapter dis-cusses these interactions, their consequences, andpossible interventions to prevent them.

WORK SCHEDULES

The characteristics of work schedules that can bevaried include the length of the work period, theplacement of the work period in the 24-hour day, theregularity of that placement, the speed at which theplacement changes, and the ratio of work time to resttime. Any schedule that requires workers to workwhen they would normally be sleeping (andsleeping when they would normally be awake)can disrupt circadian rhythms (31). This includesschedules that require workers to constantly changethe hours that they work or to work for extendedperiods of time.

Shift work is not new (103,144). Historically,bakers have worked through the night to ensure freshbread in the morning, and in ancient Rome deliverieswere restricted to the night hours in order to relievetraffic congestion. With the initiation of the Indus-trial Revolution, more and more work processesrequired fill-time operation. Since then, there hasbeen an increase in the prevalence of shift work asindustrial needs for 24-hour operations combinedwith more and more service industries providingevening or around-the-clock coverage.

Thus, the person will frequently be changing thescheduled hours he or she works. In a fixed shiftschedule, the worker always has the same workhours.

Work that continues for long periods of time(i.e., usually beyond 12 hours) and causes work-ers to decrease or miss their normal sleep isconsidered extended duty hours. Often there is nota clear distinction, or there is overlap, between shiftwork and extended duty hours. For example, work-ers on a rotating shift schedule may work overtime,exposing them to both extended duty hours as wellas the rotating shift. Extended duty hours can lead todisruptions in circadian rhythms that interact withother factors, especially loss of sleep and fatigue, toaffect health and performance. (Sleepiness is theinclination to sleep, whereas fatigue is weariness dueto physical and mental exertion.) It is possible toexperience one without the other, but both, eitheralone or in concert, can have deleterious effects. Thisis especially true if such work schedules are consis-tently marked by extended work periods inter-spersed with relatively short rest periods and areirregularly scheduled. Examples of occupations thatinvolve extended duty hours are medical and surgi-cal residencies at hospitals, military operations,long-haul trucking runs, and any setting in whichthere is frequent overtime work.

As described in chapter 4, a variety of shiftschedules are used in work settings, includingschedules of fixed or rotating shifts. In a rotatingshift schedule, the individual works one shift fora period of time before rotating to the next shift.

–87–

88 . Biological Rhythms: Implications for the Worker

METHODOLOGYBecause the effects of work schedules are

sometimes subtle and involve a number of fac-tors, they can be difficult to study. Frequently, themost important variables are those that occur outsidethe workplace (e.g., quality of sleeping during theday, social effects), thus research relies to a largeextent on self-report and anecdotal information.Three basic classes of studies have been used toexamine effects of work schedules: 1) field studies,2) survey studies, and 3) laboratory studies.

Field studies, in which the researcher studiessubjects in their actual work environment, arecomparatively rare in the world literature andlargely absent from the U.S. literature because ofthe high cost and difficulty of getting substantivephysiological, production, or subjective measuresfrom a group of workers for a reasonable period oftime (e.g., one complete cycle of a rotating shiftschedule). Field studies often have a small samplesize and require a dedicated group of volunteersubjects if circadian rhythm or sleep variables are tobe measured properly. Such studies can provide aninvaluable picture of what is happening to the sleep,circadian rhythms, and performance of shift workersunder various conditions. An example of fieldstudies that have been carried out in the UnitedStates are those conducted by the National Aeronau-tics and Space Administration (NASA) on fatigue,sleep, and circadian rhythms in flight crews. Fieldstudies in other occupations have been carried out inSweden (5), West Germany (84), the United King-dom (158), and France (129).

Physiological measures taken in field studies aretypically in the domains of circadian rhythms andsleep. Body temperature is the preferred variable inmeasures of circadian rhythms because it is easilymeasured and reliable in indicating the status of thecircadian system. Ideally, sleep is measured bydirectly monitoring physiological functions (e.g.,brain activity, eye movements, muscle activity)during sleep, but this is often impractical, sosubjective measures, such as sleep diaries or sleepquestionnaires, are used. Another method, which hasbeen used more recently, is wrist activity monitorsworn by workers to collect information on activitylevels, from which amounts of sleep are inferred

(128). Mood, social and family factors, and perform-ance can be measured by self-report tests, sometimesaugmented by measures of on-the-job performance.

Survey studies are considerably easier to con-duct and usually involve questioning the workerin one or two interviews or classroom-typesessions. These data are augmented by data frompersonnel, health service, and absenteeism files anda review of production figures. Neither circadianrhythm nor sleep diary measures can be obtaineddirectly in survey studies. Data on these variables aregleaned from questions about perceived levels ofalertness while on duty and average timing andquality of sleep after various shifts. Survey studiesare often carried out before and after an interventionin order to assess its impact (36). Contamination b yplacebo effects,l which can endanger the validity ofoutcomes, is a problem that needs to be carefullycontrolled for in survey studies (116).

Laboratory studies attempt to simulate theworkplace in a controlled situation. In some cases,workers are used as subjects, but since that is oftenimpractical, studies usually use other individuals assubjects (86,175). The major advantage of labora-tory studies is that they are the best way to getaccurate, clear, and complete recordings of sleep andcircadian rhythms (and mood and performancemeasures) without all of the interference and missingof readings that are so characteristic of field studies.The disadvantage is that some factors that play a rolein the real work situation (e.g., social and domesticstrains, the length of exposure to the schedule beingstudied) and the types of stress experienced may notbe replicated exactly. Another important considera-tion in laboratory studies is the nature of the task thesubject is to perform. In some cases, when generalinformation about the effects of a work schedule onvariables such as performance, alertness, and vig-ilance is desired, standardized tasks and tests can beused. However, in studies of the effects of a scheduleon a specific type of job, simulators are used torecreate the work environment. For example, simu-lators that replicate an aircraft cockpit, a nuclearpowerplant control room, or an air traffic controltower are effective aids in the study of the effects ofwork schedules on worker performance duringspecific job-related tasks.

l~e place~ effect is when an htervention that has no real, direct action results in an effecti e.g., when a subj=t reports fee~g better after ~v~gbeen given a sugar pill. The effect is due not to the intervention but to the subject’s anticipating an effect from the intervention.

Chapter 5--Biological Rhythms and Work Schedules ● 89

STRESSORS CAUSED BY WORKSCHEDULES

Human beings are essentially diurnal creatures,whose internal clocks are naturally geared towardsleep and inactivity at night and activity andwakefulness during the day. Because of this, the vastmajority of human society is structured with theexpectation that work will be done during the day,recreation in the evening, and sleep during the night.Morning and evening rush hours are catered to bymass transit; prime-time television, sports events,and social gatherings occur in the evening; and aquiet environment is enforced at night. Socialtaboos, for example, protect a day worker’s sleepfrom nonemergency telephone calls at 2 a.m., butnone protects a night worker’s sleep from 2 p.m.calls. Thus, the shift worker is very often fightingboth the natural diurnal trend and socioculturalattitudes.

Consequently, there are three sources of stress, orstressors, for the shift worker:

~ disruption of circadian rhythms,. disruption of sleep and fatigue, and● social and domestic disturbances.

These stressors act alone and in combination toproduce adverse effects on the worker. The first twoare physiological effects associated with the hours ofwork and the length of the work period. The third canresult from the worker’s interactions at home andwith the rest of society as a result of the workschedule. How much stress these three sourcesplace on the worker varies with the nature of thework situation and the work schedule. For theperson working extended duty hours, sleep loss andfatigue may be the most salient factor, while for therotating shift worker, circadian disruptions may beas important.

The impact of these stressors and their resultantconsequences may, in some individuals and somesituations, lead to difficulties in coping with thework schedule. There is great variability amongpeople in their ability to adjust to shift work, withsome individuals suffering few, if any, problemsand others finding certain work schedules intol-erable. It should be kept in mind, however, that eventhose individuals who appear to adjust well to shiftwork may experience negative effects on factorssuch as performance or safety. Also, for some indi-

viduals, some of the nonphysiological characteris-tics of shift work (e.g., improved access to daytimeservices, more time with young children, a some-times lighter workload, higher pay) may be positiveattributes. For others, the negative factors associatedwith shift work make adaptation difficult. In a studyof 9,000 shift workers, it was found that 20 percenthad difficulty adapting to shift work (140). Thispercentage rises as one moves to older age groups.In a study of an Austrian oil refinery, it was foundthat more than 80 percent of the sample expected tobe unable to continue shift work until retirement(88).

Disruption of Circadian Rhythms

As previously mentioned, work schedules canhave two consequences related to circadian rhythms.First, a schedule may require that an individual beawake and active at an inappropriate time during thecircadian cycle. Second, work schedules can resultin a state in which the individual’s circadian rhythmsare out of synchrony. As described in chapter 3, thecircadian system is kept on track by an entrainmentmechanism that uses time cues in the environment(light-dark) and the behavior of the individual(social interaction, meal times, sleep-wake schedule,and so on). This entrainment mechanism keeps anindividual’s circadian cycles aligned and in tunewith the 24-hour rotation of the Earth. The processof adjustment to the abrupt, large changes in routinecaused by shift work is slow, typically taking a weekor more (82,108,175) (figure 5-1). Not only does theinherent inertia of the circadian system slow adapta-tion to a work schedule, the presence of competingtime cues (e.g., light-dark. social cues) further slowsthe adjustment process.

Irregular work schedules (e.g., those routinelyexperienced by locomotive engineers) and schedulesthat involve frequent crossing of time zones (trans-meridian airline crews) result in chronic circadiandesynchronization. In rotating shift work, althoughthe rotation schedule is freed and regular, the lengthof time spent on a given shift may not be sufficientto allow for complete resynchronization of thecircadian system. In weekly rotations, completere-entrainment may never occur, and the worker maybe in a perpetual state of circadian desynchroniza-tion (150). In a rapidly rotating schedule (1 to 3 days)shift changes occur so quickly that the circadiansystem does not have enough time to begin resyn-chronizing before the next rotation change; as a

90 ● Biological Rhythms: Implications for the Worker

result, the worker’s cycles remain diurnal (figure circadian cycles never actually takes place in shift5-2). However, the individual will be out of syn- work due to the competing influences of cues in thechrony when he or she is working on the night shift. society telling workers that they are on an abnormalSome researchers (56,82) maintain that, regard- schedule. Even in permanent night shift work, if theless of rotation length, complete realignment of worker does not maintain the same schedule on days

Figure 5-l—Relationship of Body Temperature, Work, and Sleepon a Permanent Night Shift

1 Sleep Work Sleep

37.5

37.0

36.5

Reference day36.0 I I 1 I I 36.0 / 3rd night shift

1 I I I

37.5

37.0

36.5

6 am Noon 6 pm Mid- 6 amnight

I Sleep Work

\

— — ———

36.01st night shift

I 1 I r 16

37.5I

37.0

36.51

am N o o n 6 pm Mid- 6 amnight

Sleep Work

36.02nd night shift

I I 1 I 16 am Noon 6 pm Mid- 6 am

night

37.5

37.0

36.5

36.0


Sleep Work

7th night shift-~

6 am Noon Mid- 6 pm 6 amnight

I Sleep Work

37.5

37.0

36.5I

36.021st night shift

I I I I I


Time of day

Graphs showing average body temperature of workers on the day preceding the start of night work andon days on the night shift. By the seventh night, body temperature has synchronized to the nightschedule.SOURCE: Adapted from S. Folkard, D.S. Minors, and J.M. Waterhouse, “Chronobiology and Shift Work: Current

Issues and Trends,” Chronobio/ogia 12:31-54, 1985.

‘(


38.0

37.5

37.0

36.5 1

Figure 5-2—Relationship of Body Temperature, Work, and Sleepin a Rapidly Rotating Schedule

Work Sleep— -— — .

1st morning shift

6 am Noon 6 pm Mid- 6 pmnight

Work Sleep—— — — -

3 8 . 0 -

3 7 . 5 -

3 7 . 0 -

36.5- 2nd morning shift1 I I I


Sleep Work— —————

38.0-

37.5-

\37.0-

36.5 1st afternoon shift

I I I

Sleep

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1 ---6 am Noon 6 pm Mid- 6 am

night

Sleep Work Sleep— — ——— +

38.0-

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37.0-

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38.0

37.5

37.0

36.5

Sleep

1st night shiftI I I

6 am Noon 6 pm

I Sleep

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\T

Mid- 6 amnight

Work

I n ~-– – –-1

1 zna nlgnl snl36.5 + ,I I A : . , . , .ft’I 1

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night

Sleep Sleep

38.0-

37.5-

37.0- UL

36.5- 1st day offI I 1 I


38.0

37.5

37.0

36.5


Time of day

Graphs showing average body temperature of four workers on a 2-2-2 rotating shift system. On the nightshift, the low point of body temperature coincides with the work period.SOURCE: S. Folkard, D.S. Minors, and J.M. Waterhouse, “Chronobiology and Shift Work: Current Issues and Trends,”

Chronobiologia 12:31-54, 1985.

297-935 - 91 - 4 QL 3


off as on workdays, he or she will rapidly revert tothe natural diurnal orientation (105,165). In suchcircumstances, the circadian system never fullysynchronizes to the night schedule.

A desynchronized circadian system produces anumber of effects on the individual. First, since thecircadian system sets the time for sleep and wakeful-ness, a disrupted circadian system will interfere withboth sleep and daytime functioning. Daytime sleepwill be disrupted by daytime mechanisms, and theability to stay awake at night will be impaired bycircadian functions that are normally associated withsleeping (5,7,56). In some tasks, particularly monot-onous ones such as driving, vigilance, monitoring,and quality control, this may lead to decrements inperformance that compromise productivity and safety(42,57,108). Second, as discussed in chapter 3, theability to perform certain types of tasks is also on acircadian cycle. A desynchronized circadian systemresults in the person’s attempting to perform certaintasks at a time in the circadian cycle when his or herability to perform those tasks is not optimal (56).Finally, an inappropriately synchronized circadiansystem is in a state of disharmony akin to that of asymphony orchestra without a conductor. The dis-harmony itself may result in feelings of malaise andfatigue, as well as certain gastrointestinal symptoms(37,140).

The degree to which individuals are affected bycircadian disruption varies (132). There is often aninteraction between aging and circadian rhythms,which would indicate a greater susceptibility torhythm disturbances with age (see ch. 3) (80). Thesefactors suggest that some people may be betterable to tolerate the circadian desynchronizationassociated with shift work than others.

Sleep Disruption and Fatigue

Schedules that require work during nighttimehours are associated with sleepiness and fatigue.About 60 to 70 percent of workers on rotating shiftscomplain of sleep disruption (141), and generalfatigue is reported more frequently by shift workersthan by day workers (5). In a study of diverseoccupational settings that included steel workers,meteorologists, and police, 80 to 90 percent of thesubjects reported that they experienced fatigueduring night work (6). Sleepiness is generallyassociated more with the night shift than themorning and afternoon shifts. The most common

complaint is the inability to sleep as long asnecessary during the day (2). Permanent nightworkers also have shortened sleep, although to alesser extent than rotating shift workers (38,172)(figure 5-3). Sleep disturbances and fatigue canoccur as a result of the irregular hours typicallyworked by railroad engineers (154,159), the workschedules and jet lag experienced by commercialairline pilots (64,67), the watch schedules aboardmerchant ships (142), and the extended on-callschedules of resident physicians in hospitals. Anyjob involving long work periods, whether as a resultof overtime or as part of the regular schedule, canresult in sleep loss and fatigue (box 5-A).

A worker’s sleep can be disrupted by bothendogenous and exogenous factors. As mentionedearlier, the endogenous factors stem from thedesynchronized circadian system, which has failedto prepare the body and mind for sleep and wakes thesystem up before 7 or 8 hours of restful sleep havebeen obtained (5,56). Often, workers will complainof being wakened by exogenous factors (e.g., trafficnoise, children playing), whereas in reality it is theirdesynchronized circadian system that is the culprit(4,59). Workers will also experience fatigue andsleep loss from any work schedule that requires themto stay up for long hours.

Exogenous factors can be just as big a problem.The daytime environment is much less conducive tosleep than the nighttime. Daylight can intrude on

Figure 5-3-Length of Sleep of Permanent andRotating Shift Workers

Hours8 1

6 -

4 -

2 -

0’A B c D

= Day = Afternoon, evening m Night

Average sleep lengths on different shifts for two samples ofpermanent (A,B) and rotating (C,D) shift workers. Rotating shiftworkers on the night shift had the shortest average sleep lengths.SOURCE: M. Colligan and D. Tepas, “The Stress of Hours of VWk,”

Ameiican Indstrial Hygiene Assoa’athn Journal 47;S86-695,19s6.

Chapter 5--Biological Rhythms and Work Schedules . 93

Box 5-A—Nigbt Shift Paralysis

One consequence of sleep deprivation is the occurrence of a ram phenomenon known as night shift paralysis.This condition is marked by a momentary paralysis, usually lasting about 2 minutes, during which individuals areaware of their surroundings but are unable to make voluntary movements. Night shift paralysis has been observedin studies of nurses, naval officers, printers, and air traffic controllers and is associated with high levels of sleepiness.

In a study that looked at night shift paralysis in 435 air traffic controllers from 17 countries, 26 claimed tohaveexperienced night shift paralysis at some time during their careers. A total of 75 incidents was reported by these26 controllers. The researchers conducting the study calculated that the potential for an accident resulting from anoccurrence of this paralysis is 0.5 time in each individual’s working life. No accidents or near misses were associatedwith any of the episodes of night shift paralysis reported in this study.

Factors identified as contributing to the occurrence of night shift paralysis include time of night, the numberof consecutive night shifts the person had worked, and having worked both a morning and a night shift on the sameday. Of the 75 reported incidents, 56 took place during the night shift, 12 during the day shift, and 7 during themorning shift. Interestingly, it was observed that individuals who were more rigid in their sleeping habits were morelikely to experience night shift paralysis than persons who were able to fall asleep at unusual times. This seeminglyparadoxical finding maybe due to the fact that flexible sleepers may suffer less sleep deprivation when workingshifts (see text),

The researchers concluded that the incidence of night shift paralysis maybe a useful reflection of the extentof sleep deprivation. They also suggested that its occurrence maybe reduced by limiting the number of successivenight shifts to one and by not allowing an individual to work both a morning and a night shift on the same day, Thus,while night shift paralysis is an extremely uncommon event, it does illustrate how work schedules can cause sleepdeprivation, with potentially far-reaching effects.SOWRCl% S. Follrard and R. Condoni “Night SMt Paralysis in Air Traffk Control Clf33cers,” Ergonomics 30:1353-1363, 1987.

sleep if the sleeping quarters are not sufficiently morning or afternoon shifts, but depending on thedarkened. Households are noisier, and there is amuch greater chance of interruption from telephonesringing, televisions playing, and general activityboth inside and outside the home. In addition, theworker may feel compelled to interrupt sleep volun-tarily in order to meet family and social demandsthat occur during the day (101,153). Consequently,day sleep is much more susceptible to disruption bydomestic commitments (shopping, child care, and soon) than is night sleep. Very often, social anddomestic disharmonies (see later discussion) tend todevelop because family members get blamed forsleep disruptions that may not be entirely their fault.In addition, if a shift worker takes on another job, thelikelihood that he or she will set aside adequate timefor sleep diminishes further.

The net effect of these sleep disruptions is lackof sufficient sleep, which often results in a state ofchronic fatigue and sleepiness referred to as sleepdebt (5). This is especially true for rotating shiftworkers while they are on the night shift, permanentnight workers, and individuals who are routinelysubjected to long and irregular hours of work thatextend into the night. For rotating shift workers,there may be some respite while they are on the

design of the schedule, the degree to which theyrecoup the sleep lost during the night shift will vary.Sleep debt has important implications for workerperformance, safety, and health (figure 5-4).

Individuals vary in their ability to sleep. Somepeople are more rigid in their sleep habits and havetrouble sleeping at unusual times, while others seemto be more flexible in their ability to sleep at oddtimes. Some evidence suggests that people who findit easier to sleep at odd times may have an easieradjustment to the disruptions in sleep that can occurfrom work schedules (33,57).

Social and Domestic Disturbances

Work schedules may induce stress by prevent-ing the worker from fulfilling important familyroles (167). Social companionship, parenting, andsexual partner roles can all be compromised by workschedules. These effects may be major and canseverely affect mood, motivation, and sleep, there-fore having indirect effects on performance andsafety. Marital problems, excessive domestic load,and community alienation have all been documentedas a result of the strain placed on workers by work


Figure 5-4-Sleep Duration and Fatigue in AircrewsDuring a 4-Day Trip

A.Sleep duration (hours)

87

7.5-

7 -

6.5-

6 -

5.5-

5Pretrip Trip

B

T

Posttrip

T

schedules. A survey of 1,028 married couples in theUnited States found that shift work is associatedwith lower-quality family time and more frequentfamily conflicts (148). Another study found thatshift work increased the probability of divorce from7 to 11 percent over the 3-year period of the study(178). A more dramatic effect was reported in astudy of 1,490 workers (152) in which an almost 50percent increase in divorce and separations wasfound among those on freed night shifts compared tomorning and evening shifts. When asked whetherthey were satisfied with the amount of time they hadavailable to spend with family and friends, workerson night or evening shifts were about half as likelyto respond “yes” as their morning shift colleagues.Similar findings come from Europe and Scandina-via. In a 15-year study of 504 Swedish paper millworkers (87), the divorce rate among shift workerswas double that of day workers.

Excessive domestic workload represents a majorsource of stress from shift work for women workers.Married women are often expected by their hus-bands to continue to run the household, raise thechildren, and do domestic chores; shift work can

/

compound this burden (63). For these women thereis usually insufficient free time available for house-hold duties, and time is therefore borrowed fromsleep. It has been observed that female night workers

I I I I1 2 3 4

Day of trip

A) Self-reported sleep duration of aircrews on a 4-day trip and the? days preceding and following the trip. B) Subjective ratings of

fatigue during the trip.SOURCE: Adapted from R.C. Graeber, “Aircrew Fatigue and Circadian

Rhythms,” Human Factors in Aviation, El. Weiner and D.C.Nagel (eds.) (San Diego, CA: Academic Press, 1988).

with - two children slept about 9 hours less per weekthan their unmarried female colleagues (63).

With the increase in dual-income families, a morecommon mode of arranging child care is for spousesto work different shifts (113,125). This splitting ofchild-care responsibilities between two workingindividuals also extends to nonspouse care givers. Astudy showed that one-third of grandmothers whoprovide child care for their grandchildren, and whoare employed, work a different shift than thechildren’s mothers (126).

Finally, social problems are encountered byshift workers. Society is geared toward a Mondayto Friday, daytime work schedule. Several research-ers (167) have found that shift workers may feelalienated from the community because they areunable to attend evening educational, sports, orreligious meetings or weekend recreational events.Team or club membership becomes difficult becauseof irregular attendance, and few shift workers holdoffice in clubs and. societies. The shift worker canfeel left out of community benefits and opportuni-


Photo credit: David Liskowsky

Obtaining adequate sleep during the day is often difficultfor the shift worker.

ties. Such effects are much less frequent in companytowns, where shift work is the rule rather than theexception, and as a consequence nonstandard workschedules are considerably better tolerated (174).

CONSEQUENCES OF WORK-RELATED STRESSORS

The three stressors previously described (cir-cadian rhythm disruption, sleep disruption, socialand domestic disturbances) can combine to cause avariety of problems for the worker. The degree towhich a person suffers from the consequences ofwork-related stressors varies greatly among indi-viduals. At the present time, not enough is known tomake a definite statement about who will or will notbe able to cope with shift work. As was mentionedearlier, the ability of a person to adjust to circadiandisruption and sleep disturbances may affect his orher ability to tolerate the demands caused by work

schedules. For example, circadian rhythm patternsare not the same in all people. Some people haveconsistently longer free-running periods and a wideramplitude between their maximum and minimumdaily body temperatures. Evidence suggests thatthese individuals are better able to tolerate shift workthan persons with naturally shorter free-rumingperiods and a narrower body temperature range(20,129,130,131). However, it is not clear if thesecharacteristics are a benefit in all shift work sched-ules.

Other characteristics related to sleep habits andpersonality have been combined to divide peopleinto two categories. It has been hypothesized thatthese categories may have some validity in predict-ing success in coping with work schedules. Amorning person is someone who gets up easily andis active in the morning, has a hard time sleepinglate, and falls asleep quickly in the evening. Incontrast, an evening person is more alert at night,able to sleep late in the morning, and takes a longtime to fall asleep at night. It is thought that theevening person is more adaptable to varying workschedules (106,1 10,120). Additional information isneeded to test the validity of this hypothesis and toidentify more clearly variables that might makesome individuals better able to cope with changingwork schedules than others.

Another important factor in determiningg who willcope best with shift work is age. As one ages, theinternal clock and sleep patterns change (figure 5-5).The internal clock may become harder to reset withage, and sleep becomes more fragile and easilydisrupted. These effects usually begin to occur afterthe age of 45. For example, a survey of rotating shiftworkers found that older workers (45 to 60 years old)had more difficulty adjusting to night and afternoonshifts than did younger workers (79). It is notuncommon for a person who has been working shiftsall of his or her career without undue strain to starthaving trouble coping as he or she gets older. Avariety of health problems can appear with age andcan add to the difficulty of coping with shift work(109).

While these factors suggest who might be bestsuited for shift work, the combined effects ofthese and other factors (e.g., physical ailments,family and social status) increase the variabilityof individuals’ ability to cope with shift work (90).As a result, for some people, shift work is only an


Figure 5-5-Length of Sleep of PermanentNight Shift Workers

420-

4oo -

380-

360-

*

3401 I

3204 1 I 1 118-29 30-39 40-49 50-59

Age group

Graphs showing the average sleep length for male and femalepermanent night shift workers, according to age.SOURCE: D. Tepas, Department of Psychology, University of Connecticut,

1990.

inconvenience, while for others it can have majorconsequences. It has been estimated that about 20 to30 percent of shift workers actively dislike theirschedule (56). Often, workers who find they cannotadjust to the demands of shift work change to dayjobs. Thus, there is a strong self-selection process inoperation that has important implications for studiesof the effects of work schedules. The populationsbeing studied may represent individuals who areindifferent to or who can tolerate the negativeconsequences of shift work, and thus the studies mayunderestimate the true effects of nonstandard sched-ules (11 1).

Health

The stressors described in the previous sectionmay interact to have negative effects on the health ofa worker. In general, these effects result fromchronic, repeated exposure to stressors, for example,the continuing circadian desynchronization thatrotating shift workers experience or the constantstress placed on night workers to meet family andsocial obligations during the day. Surveys haveshown that shift workers, particularly those onrotating shifts, have a higher incidence of sick leave,a higher rate of visits to clinics at the work site, andpoorer scores on a variety of indexes of health(11 1,112). Thus it appears that a greater propor-tion of shift workers than day workers suffergeneral health complaints. For example, a study of

policemen found that those working rotating shiftsreported a higher incidence of complaints of muscleaches, respiratory infections, and gastric complaintsthan did officers working days (123). In addition togeneral health complaints, there is some relationshipbetween shift work schedules and the occurrence ofspecific disorders, primarily gastrointestinal prob-lems.

Shift workers, particularly those on night shifts,commonly complain of gastrointestinal disorders(94,111), including general gastric discomfort andpeptic ulcer disease (15,37,111,140). One studycomparing the incidence of peptic disease in nightworkers and in day workers showed that the twogroups begin to differ only after 5 years of exposureto their respective work schedules (10). There areprobably several reasons for the higher incidence ofgastrointestinal complaints among night workers.Intestinal enzymes are secreted in a circadian cyclethat is disrupted by night or shift work schedules,contributing to digestive distress. Also, the linkbetween meal patterns and the circadian system,which has to suspend appetite and voiding in orderto allow 7 or 8 hours of uninterrupted sleep, mayplay a role. For example, the eating habits of shiftworkers tend to be irregular, with workers notadjusting their meal schedules to match their workschedules (47,56,72,120). It is also often difficult toobtain high-quality meals at odd hours (56) (figure5-6). The link between eating habits and gastrointes-tinal complaints in shift workers indicates an area inwhich employee education might play a useful role.Risk factors associated with gastrointestinal disor-ders, such as increased alcohol and tobacco con-sumption, have also been shown to be higher in shiftworkers than in day workers (10,149). Finally, thepsychological stress associated with family dis-harmony or social disruption could contribute togastrointestinal problems.

Another major medical concern is cardiovasculardisease; however, the link between it and shift workis not as strong as that for gastrointestinal problems.While some earlier reviews of the field did not findcompelling evidence of a link (72,140), more recentresearch from Sweden (87) has demonstrated thatshift work is associated both with increased risk ofcardiovascular disease and with its associated riskfactors (122). A study of Akron police officersshowed that rotating shift work may be associatedwith high levels of norepinephrine (a body chemicalthat causes an increase in heart rate and blood

Chapter 5--Biological Rhythms and Work Schedules .97

Figure 5-6-Shifts Interfering With Eating Habits

80

7060 150{40]302010 \nL

Day Afternoon Night

Shift

The percentage of coal miners reporting that a given shiftinterfered with their eating habits.SOURCE: J.C. Duchon and C.M. Keran, “Relationship Among Shiftworker

Eating Habits, Eating Satisfaction, and Self-Reported Health ina Population of Miners,” Journa/of Work ard Sfress 4:1 11-120,1990.

pressure), which could place workers at higher riskof cardiovascular problems (54). A recent review ofthe literature related to cardiovascular disease andthe work environment concluded that ”... the betterstudies in the field consistently find a modestlyhigher incidence of cardiovascular disease amongshift workers” (89).

Finally, a few studies present data suggesting aslightly increased risk of miscarriage, preterm birth,and lower birth weight among women who workrotating shifts or irregular hours (12,14). A study ofcomplications during pregnancy and adverse out-comes of pregnancy found no link to shift work, butit did note an association between shift work and atendency for babies to be smaller for their gesta-tional age (1 19).

Beyond these specific disorders, the adverseeffects of shift work on workers’ health seem to bediffuse, affecting some workers’ general sense ofwell-being. These workers frequently report sleepdisturbances and fatigue, menstrual problems, in-creased feelings of irritation and strain, increaseduse of alcohol and other drugs (tranquilizers, caf-feine), and a general feeling of malaise that maybeexacerbated by psychological stress related to beingless satisfied in the domestic and social areas of theirlives (65,112). It has also been proposed that thestresses associated with shift work might exacerbate

preexisting health conditions, increasing their sever-ity without increasing their incidence (29).

A related health problem maybe the inability ofthe shift worker to comply with health and medica-tion regimens. As discussed in chapter 3, the body’smetabolism and response to drugs are generallycircadian. Also, symptoms of some disorders, suchas diabetes, depression, and asthma, evidence signif-icant circadian fluctuations. A shift worker could bein the position of taking medication at inappropriateor constantly changing times. Also, variable workschedules could make it difficult to coordinatetaking medication with the work-rest cycle andmealtimes, thus interfering with the worker’s abilityto take medication at all (29).

In summary) shift work schedules are associatedwith increased gastrointestinal complaints amongworkers and may be a risk factor in cardiovasculardisease and such pregnancy outcomes as low birthweight and preterm births (figure 5-7). Beyond thesespecific ailments, the health effects of such workschedules generally manifest themselves as com-plaints of decreased well-being, chronic malaise,and poor sleep. Shift maladaptation syndrome is theterm used to describe the effects of long-term shiftwork on the health of workers who have never beenable to adjust to it.

Performance

The effects of work schedules on performance arepart of a complex set of factors that shape a person’sperformance in a given situation. These factorsinclude the circadian, sleep and fatigue, and socialand domestic stressors already described. They alsoinclude the type of task to be performed (e.g.,vigilance, physical, cognitive), motivational effects(which are significantly influenced by social andfamily concerns), the work schedule being em-ployed, and individual differences among workers(e.g., personality, age, health, sleep needs, behaviorpatterns) and how they adjust to changes in routine.Unlike health effects, performance decrements mayoccur soon after exposure to a work schedule.

As discussed in chapter 3, performance on anumber of different types of tasks displays acircadian rhythm, and different types of performancediffer in their normal phase and the degree to whichthey are affected by outside influences. For example,tasks involving signal detection, reaction time, andhandling of simple arithmetic correlate with cir-


Figure 5-7—Prevalence of Disease in Shift Workers and Day Workers

Percent

60

40

20

0

Percent

6 0 –

40-

20-

0-Workexperience(years)

Shift workers

[

Day workers

dl0-3 4-12 13-2223-40

Circulatory system

0-3 4-1213-2223-40

Digestive system

Diseases

Percent of shift workers and day workers at an oil refinery, subdivided into groups according to workexperience, suffering from circulatory and digestive diseases. Shaded bar indicates that there was astatistically significant difference between the shift workers and day workers.SOURCE: Adapted from M. Keller, “Health Risks Related to Shift Work: An Example of Time Contingent Effects of

lxmg-Term Stress;’ International Archives of Ocwpationaland Environmenfai Health 53:59-75, 1983.

cadian changes in body temperature, peaking duringthe afternoon, while performance of cognitive tasksinvolving memory may peak in the morning (28,56).Performance can be directly affected by circadiandesynchronization and inappropriately timed activ-ity, as well as by the loss of sleep they can cause.There is a large body of literature regarding theeffects of fatigue and sleep deprivation on perform-ance; these factors clearly have a negative effect onthe performance of most tasks (28,91).

In contrast to the fairly extensive research onthe circadian component of performance and theeffects of sleep deprivation on performance,much less research has been done on the complexinteraction of variables that occurs in a worksetting that can also affect worker performance.To determine the effects of work schedules onperformance, studies can examine the performanceof a worker either on a standardized test battery afterbeing exposed to a certain work schedule or on the


actual task the job involves. A factor that cancomplicate the study of intershift differences in taskperformance and perhaps mask the effects of shiftwork is the difference in working environments. Notonly environmental conditions, such as lightinglevels, but also supervision levels, group morale, anddistractions can all vary between night shifts and dayshifts. Also, poorer performance can occur on thenight shift simply because no one is available torepair broken machines (102). The work itself maybe quite different on the night shift. Very often,certain parts of the job are saved for the night shift,either because the process demands it (preparingthings for shipment in the morning) or because itplaces less demand on the night workers. In contin-uous-process operations, complicated developmentwork may intrude during the day but not at night(40).

The relatively few studies that have recorded24-hour real-task data in the field demonstratedecreased performance at night. They have shownthat the speed at which a task can be performeddecreases at night (21,182) and the probability ofmaking an error, missing a warning signal, ornodding off while driving is highest at night(17,58,75,127) (figure 5-8). In addition, some evi-dence shows a pronounced postlunch dip in thesemeasures. More recent studies have found the risk ofaccidents involving truck drivers between midnightand 2 a.m. to be more than double the average duringthe day (69). Job performance of nurses, as measuredby a questionnaire filled out by supervisors, wasfound to be lower in those on a rotating shift than inthose on freed day, afternoon, or night shifts (27).

Studies using other measures of performance havealso found effects related to work schedules. Forexample, results of a study that measured manualdexterity and search performance of 10 factoryworkers on a rotating shift showed the workersperformed worst on the tests when working the nightshift (181). The tests were administered just beforethe start of a shift and after 4 and 8 hours of work.Another study brought shift workers into the labora-tory during nonwork hours for performance testingand measures of brain activity during sleep. Theseworkers continued to work at their normalworkplace, but they commuted to and from thelaboratory rather than home (155,156). Their per-formance on a vigilance task was examined justbefore bedtime, at bedtime, and on awakening. Thestudy involved 10 night workers, 10 day workers,

Figure 5-8-Work Performance in VariousJob Settings

Time of day

Mid-8 am Noon 4 pm 8 pm night 4 am 8 am

I I 1 I r I I , I I

I 1 1 I I I 1 I , I I

3 am Noon 4 pm 8 pm Mid- 4 am 8 am

Fast

Speed ofansweringswitchboard

slowLow

Error frequencyin reading meters

High

Low

Frequency of“nodding off”while driving

High

Fast

Speed of joiningthreads (spinners)

slowLow

Frequency oftrain driversmissing warningsignal

High

Low

Frequency ofminor accidentsin hospital

High

night

Time of day

Variations in measures of on-the-job performance, by time of day,from a variety of studies.SOURCE: Adapted from T.H. Monk, “Shiftworker Performan-,” Shift-

work, Occupational Medb’ne State-of-the-Art Revfews, A.J.Scott (ed.)(Philadelphia, PA: Hanley & Belfus, 1990).


and 10 (slowly) rotating shift workers who wereworking day or evening shifts at the time they weretested. Night shift workers performed worst on allthree performance tests, but there were no reliabledifferences between the other two groups. Thedifferences in performance were not accompaniedby equivalent differences in body temperature, butthey were correlated with brain activity during sleepin a way that is typical of subjects undergoingchronic partial sleep deprivation.

Similar results were obtained when 12 shiftworkers were studied for one complete cycle of theirweekly rotating shift in the field rather than in thelaboratory (158). Reaction speeds on both a simpleand a complex reaction-time test were slower amongworkers on the night shift. There was also strongevidence of an increasing deterioration in perform-ance, which the authors interpreted as a buildup ofpartial sleep deprivation. The effects on a number ofdifferent measures of performance of the 4-hours-on, 8-hours-off schedule typically used on merchantships for standing watch has also been examined(32). There was only a partial adaptation of circadianrhythms to this schedule, and most performancemeasures showed lower values during the nighthours. The decrements in performance were exacer-bated by disturbances in sleep that occurred as aresult of the split hours of sleep time necessitated bythis schedule. These studies demonstrate some of theeffects that disrupted circadian rhythms can have onperformance and their interaction with other factorsinvolved in shift work, notably sleep deprivation.

The studies described above examine the effectson performance of work schedules involving rotat-ing shifts or night shifts. Jobs that require sus-tained or extended duty hours can also affectperformance. For example, jobs such as cross-country truck driving require long hours of constantattention, often in a boring or repetitive environ-ment, while jobs such as those of residents inhospitals involve continuous work, interrupted byperiods of reduced activity or rest breaks. Whilecircadian disruption plays a role in the performancedecrements observed under these conditions, sleeploss is probably an equally salient factor (91).

A laboratory study that required subjects toperform 54 hours of continuous work found thatreaction time, logical reasoning, vigilance, andencoding and decoding messages all declined in astepwise fashion; that is, declines in performance

were interspersed with plateaus in performance (11).The declines in performance were paralleled bysubjective ratings of fatigue and sleepiness; thegreatest declines occurred in conjunction with thelowest point in the circadian cycle of body tempera-ture. Similar results have been described when acomputerized test battery was used to measureperformance (115). While these studies show thatdecrements in performance can occur as a result ofextended duty hours, a number of other studiesfound that certain kinds of performance are moresensitive to sustained operations than others (91). Ingeneral, these studies found that psychologicaltasks, such as cognitive, vigilance, long-term mem-ory, decisionmaking, a n d p e r c e p t u a l - m o t o r t a s k s ,were more likely to be adversely affected bysustained operations than were physical tasks in-volving gross motor activity and strength.

Thus it is reasonable to assert that, in general,shift work decreases performance and that anumber of factors besides circadian desynchroni-zation contribute to this decrease, notably sleepdeprivation and fatigue. A great deal more infor-mation still needs to be derived, however. As withhealth effects, there is a complex interaction offactors that under actual working conditions couldcontribute to or counteract decrements in perform-ance (namely, nature of the task, speed at which thetask is performed, work schedule employed, workerage, family and social factors, motivation). Moreinformation is needed to delineate the effects ofthese interacting factors in a given work situation. Abetter understanding of how performance on differ-ent types of tasks can be affected will make itpossible to devise interventions and counter-measures.

Safety

The factors described earlier also have an impacton worker safety. Performance decrements resultingfrom sleepiness, circadian disruption, or distractionby family problems can cause situations that mightcompromise the safety of the individual and, de-pending on the nature of the job, public safety aswell.

In the United States, the major mechanism forcollecting data related to workplace injuries andmishaps is the Bureau of Labor Statistics (BLS) inthe Department of Labor. Currently, BLS forms forreportable injuries and illnesses include no questions


regarding the time of day an incident occurred, thetype of schedule an employee involved in anincident was working, the number of hours since theemployee began work, or the total number of hoursor shifts the employee had worked over the preced-ing days. Thus, data currently being gathered bythe BLS provide no information that couldfacilitate an assessment of the impact of shiftwork on employee safety and health. The Occupa-tional Safety and Health Administration, within theDepartment of Labor, may record information ontime of day and hours of work when investigatingworkplace fatalities and catastrophic incidents inwhich five or more persons are injured or substantialproperty damage is caused.

Data on injuries can be put to three possible uses.First, they can be used to investigate whether thework environment or supervisory practices weredifferent on a particular shift and to see if schedulechanges had any effect on injury rate. Second, theycan be used for research examining features of thework environment and schedules that are related toinjury rates. Finally, the data can be used inproviding surveillance of the workplace.

Another source of data is the National Transporta-tion Safety Board (NTSB), which investigates trans-portation-related mishaps. In its investigations theNTSB routinely collects information regarding therole work hours might play in such incidents. OtherFederal agencies and departments (e.g., the NuclearRegulatory Commission or the Department of Trans-portation) may examine the role of work hours wheninvestigating incidents that occur within their do-main. For example, the U.S. Army, as part of itsArmy Safety Management Information System,maintains a database regarding Army aircraft acci-dents involving a death or loss of an aircraft (55).This database includes information related to sleepand work periods. To date, no analysis of these datahas been performed to determine the role of dutyhours in these incidents.

Other sources of information related to workschedules and their implications for worker safetyinclude surveys of specific industries or work sitesand data from other countries. As described earlier,studies of the effects of work hours are oftenconfounded by the fact that in some settings theconditions surrounding night work are quite differ-ent from those surrounding day work. This is

particularly true of industrial or manufacturingsettings.

While it is possible that the performancedecrements that occur in shift work will translateinto an increased rate of mishaps and injuries inthe workplace, the published data in this area aremeager. Some studies in industrial settings have notfound an increase in injuries related to night shifts orshift work [injuries at a chemical plant (118), U.S.coal miners (173), survey of industrial accidents inNew Zealand (49)], while others found that, al-though injury rates were higher during the day, theseverity of the injuries suffered at night was greater[iron and steel mill workers in Singapore (121), U.S.iron miners (166)]. On the other hand, a survey ofoccupational mishaps in a glass factory and a steelfactory and data on occupational mishaps occurringin Ontario over a 5-year period found that thenumber of occurrences peaked just before and afterlunch, as well as between 2 a.m. and 4 a.m. (180). Asurvey of a paint factory in the United States foundthat the injury rate was 25 percent higher for nightshift workers than for day shift workers (95). It wasalso noted that this effect was most pronounced inthe last 3 hours of the night shift. Thus, there is someindication that shift work can result in decreasedsafety in an industrial setting; however, there is nocomplete characterization of the types of settingsmost likely to be affected. Until there are moredata on the relationship between work hours andworkplace mishaps, the extent to which mishapsare caused by shift work will remain uncertainand the development of interventions to counter-act them will be hindered.

In contrast, examination of factors contributing totransportation mishaps shows the role of circadiandisruption, sleep loss, and fatigue (figure 5-9). Thesethree factors have been noted in specific incidentsinvolving every mode of transportation, includingairline, railroad, maritime, and highway driving(93). Studies have shown that the risk of a truckdriver’s being involved in a traffic collision is higherfor trips occurring at night. One study identified theperiod between midnight and 4 a.m. as the mostdangerous, with the peak being between midnightand 2 a.m. (69), while another identified the periodbetween 4 a.m. and 6 a.m. as the most crucial (100).The chances of an accident were increased further ifthe driver had been on the road or working for anextended period of time (69), and the effects ofsleepiness and fatigue were compounded if the


Figure 5-9--Fatigue-Related Vehicular Accidents

Number International data (n=6,052)1,200 I1,100; ~ I

Midnight 6 a.m. Noon 6 p.m. MidnightTime of day

The distribution, by time of day, of 6,052 vehicular accidents thatwere judged by investigators to be fatigue-related.SOURCE: M.M. Mittler, M.A. Carskadon, C.A. Czeisler, et al., “Catastr&

phes, Sleep, and Public Policy: Consensus Report,” Sleep11:100109, 1988.

driver was on an irregular schedule (100). A recentstudy by the NTSB highlighted the role of combinedsleep loss and fatigue and its interaction with druguse in a study of fatal-to-the-driver heavy truckcrashes in the United States (117). The report foundthat sleep loss and fatigue was the most frequentlycited single cause or factor in the fatal crashesexamined and that many of the drivers involved inthese incidents were also impaired by alcohol orother drugs. Another study of crashes involvinglarge trucks in the United States found that the“relative risk of crash involvement for driverswhose reported driving time exceeded 8 hours wasalmost twice that for drivers who had driven fewerhours’ (76). Similar results were found in a study ofnaval pilots who were involved in serious mishaps(destroyed aircraft, fatalities) during a 5-year period(19). Pilots who had worked at least 10 hours beforea mishap were more likely to have been at fault thanpilots who had worked less than 10 hours. Also, themishap rate was significantly related to time of day,with pilots having a lower rate of incidents duringdaytime hours.

Safety consequences of shift work can affect notonly the worker, but also the public. They have beenlisted as contributing to such highly publicizedincidents as the Three Mile Island nuclear power-plant accident and the grounding of the ExxonValdez (box 5-B). In these incidents, the strain ofextended duty hours and the time of day may have

led to errors in attention, decisionmaking, andresponse to pertinent information. Events such asthese can threaten thousands of lives and seriouslyaffect the well-being of surrounding communities.At the other end of the spectrum are less publicizedbut more common examples of threats to publicsafety, such as accidents involving interstate truck-ers. Any occupation involved with public safety andwelfare can be influenced by the effects of workschedules and their attendant problems. Decrementsin performance among police, free, and health carepersonnel could result in negative outcomes for thepublic. Additional data on the occurrence ofincidents threatening the public safety are neededto derive a clearer understanding of their fre-quency, nature, and magnitude. Such informa-tion can serve as the basis for preventive meas-ures.

INTERVENTIONSThe previous sections have described the conse-

quences and impact shift work can have in a varietyof realms. This section describes some interventionsthat may be used to lessen those effects. While anumber of different interventions are being exam-ined as possible means to counteract circadiandesynchronization, sleep disturbances, and socialdisruption associated with work schedules, defini-tive statements regarding their effectiveness cannotbe made yet. Additional research is needed on themechanisms at work in specific situations and on theeffectiveness of these interventions.

Work Schedules

Since a number of possible negative conse-quences are associated with shift work, an obviousapproach to prevention is to avoid the use of suchschedules. However, around-the-clock operationsare a fact of life in many occupations, so the bestintervention regarding scheduling is to identifythose schedules that will have the least unfavorableimpact on the individual. Making such a determina-tion is difficult, because a schedule that may be themost beneficial from a circadian perspective may notbe optimal in terms of sleep and fatigue, or aschedule that tries to avoid sleep loss may createhardships for the worker in the social or domesticarena. Also, the nature of the job and the type ofperformance it requires may make a schedulesuitable for one type of task but not suitable foranother. As stated by one researcher,” . . . the effects


Box 5-B—The Grounding of the Exxon Valdez

At about midnight on the morning of March 24,1989, the tanker Exxon Valdez, loaded with almost 1.3 millionbarrels of crude oil, ran aground on Bligh Reef in Prince William Sound, Alaska, Approximately 258,000 barrelsof oil spilled, resulting in severe damage to the environment. Damage to the vessel was put at $25 million, and thecost of the cleanup during 1989 was $1.85 billion. In the opinion of the National Transportation Safety Board(NTSB), which investigated this incident, the probable causes for the grounding of the tanker were:

s the failure of the third mate to properly maneuver the vessel because of fatigue and excessive workload;o the failure of the master to provide a proper navigation watch because of impairment from alcohol;* the failure of the Exxon Shipping Co. to provide a fit master and a sufficiently rested crew for the Exxon

Valdez;● the lack of an effective vessel traffic service because of inadequate equipment and manning levels,

inadequate personnel training, and deficient management overnight; and* the lack of effective piloting services.

This incident is an example of a situation in which negative work schedule effects combined with other factorsto bring about an event that had broad implications for the public well-being. Prominent among the NTSB findingswere a number that concerned the policies of ExxonShipping related to the manning of its tankers. TheNTSB found that these policies created conditions thatwere conducive to fatigue among tanker crews. In thecase of the Exxon Valdez, the result was that there wereno rested deck officers available to stand watch whilethe ship sailed through Prince William Sound, and theability of the third mate to corm the vessel wasimpaired by fatigue.

Among the NTSB’s recommendations related towork hours were that Exxon Shipping and other oilshipping companies eliminate personnel policies thatencourage employees to work long hours and imple-ment manning policies that prevent long workinghours. Also, the NTSB recommended that shippingcompanies forbid deck officers to combine bothnavigation and cargo watch duties using a schedule of

.// .“ , :6 hours on, 6 hours off, The NTSB recommended that

,.., ,.,.,/the U.S. Coast Guard develop means to enforce the rule ,, ,,,,/.that officers on watch during departures from port must

/

be sufficiently rested; that it expedite study programsPhoto crdii: W?. Woody, /Vatior?a/ Transportation Safety Board

to establish manning levels and safeguards to use in the The Exxon Valdez.manning review process; and that it establish manningstandards that will ensure crews are large enough to handle heavy workloads. Finally, the NTSB reiterated previoussafety recommendations to the Department of Transportation that it expedite research programs to study the effectsof fatigue, sleep loss, and circadian factors on transportation system safety; that it disseminate information andeducational material to transportation industry personnel regarding these effects; and that it review and upgradehours of service regulations in all transportation modes to ensure that they reflect the latest research on these factors.SOUR~: National Transportation Safety Board, Marine Accident Repor4 Grounding of the Zlxxon VukZez on Bligh l?eef, Prince WiZZium

sound, AK, Mar. 24,1989, NTSB/MAR-90/04 (Springfield, VA: National Teehnical Information Sewice, 1990).

of any particular work schedule are likely to be regime represents a compromise rather than anmyriad and complex, potentially exerting a differen- ideal’ (28) (box 5-C).tial influence on the various parameters of workerphysical, psychological, and social adjustment. As a The variable parameters of a schedule includeconsequence, the implementation of a specific work fixed or rotating shifts, direction and speed of


Box 5-C—The Philadelphia Police Story

In 1983, a shift rescheduling program was conducted for the City of Philadelphia in an effort to enhance thework schedules employed by the city’s police department. The two-phase project was carried out in one of the city’spolice districts. It consisted of an evaluation of the existing schedule and the design of an improved schedule (phase1), as well as the implementation of the new schedule and an education program for the officers (phase 2). (Table5-C-1 compares the old and new schedules.)

In phase 1 of the study it was found that the existing four-platoon work schedule had a number of deleteriouseffects on the officers. This included 50 percent of the officers reporting poor quality sleep; 80 percent reportingthat they fell asleep at least once a week while on the night shift; 25 percent reporting that they had been involvedin an actual or near-miss automobile accident due to sleepiness during the past year; over 75 percent of officers’families being dissatisfied with their work schedules; and 35 percent taking either an entire week to adjust or neveradjusting to the weekly shift rotation.

In phase 2 of the study, a three-platoon system was initiated for 177 officers, and educational sessions wereconducted during the implementation process. The new schedule incorporated proportional staffing (so the numberof officers on duty was proportional to the number of calls for service), reversal of shift rotation fromcounterclockwise to clockwise, reduction in the rate of shift rotation from 1 to 3 weeks, and the elimination ofregularly scheduled 6-day weeks (see table 5-C-1). Following implementation of the new schedule there was afourfold decrease in the number of officers reporting poor sleep, twice as many reporting no daytime fatigue, adecrease in the number of sleep episodes on the job, a decline in the on-the-job motor vehicle accidents per miledriven, an increase in the level of alertness on the night shill, and a reduction in the use of sleeping pills and alcohol.Families of officers reported a nearly fivefold increase in satisfaction with the new schedule.

This study shows that considering biological rhythms when designing a shift schedule for a specific worksetting can lessen the negative effects that can occur. It also points out the importance of coupling schedule redesignwith educational programs for the workers to assist them in adjusting.

Table 5-C-l—Original and Newly Instituted Schedules for thePhiladelphia Police Study

Schedule

Characteristic Old New

rotation, length of a shift, and the starting time of a (56). Also, as discussed earlier, permanent nightshift. Fixed shifts should result in the least circadian workers often suffer from chronic sleep disturbancesdisruption, assuming workers maintain the same and at-e permanently out of synchrony with the restschedule on days off as on workdays. They often do of society. This latter factor, depending on annot, however, and under such conditions permanent individual’s personality and family situation, mayshift schedules take on the nature of a rotating shift cause significant social and domestic strain.


Direction of Rotation

Based on the fact that under free-running condi-tions the human body clock has a natural tendencyto run slow, with a preferred period slightly longerthan the 24 hours of nature and society (176) (see ch.3), the human circadian system appears to adjustmore slowly to phase advances than to phase delays(13,81). From a circadian perspective, then, clock-wise shift rotation (mornings, then evenings, thennights) should be easier to cope with than counter-clockwise rotation (nights, then evenings, thenmornings). Although this hypothesis still needs to beproperly tested, there is some evidence to support it(36). However, a study that used survey data from agroup of workers indicated that, in terms of theamount of time available for sleep, a counterclock-wise schedule afforded more opportunity to preparefor and recover from the night shift (48). Further-more, it is possible that the timing of a worker’sactual sleep, which can be influenced by socialdemands, does not necessarily coincide with an idealcircadian schedule (160).

Speed of Rotation

The speed of rotation in a shift system is thenumber of days an individual spends on a particularshift before switching to a new schedule or to daysoff. There are a multitude of different shift schedulescurrently in use (85,150). Many of them approxi-mate a weekly rotation, with an employee workingfrom 4 to 7 days on a shift before changing.However, faster (1 to 3 days) or slower (more than7 days) rotating systems may also be used.

A rapidly rotating system, as noted earlier, willresult in the individual’s circadian system remainingdiurnal, since not enough time elapses for circadianrhythms to adjust to a new routine (56,105). Thisavoids the problems associated with a circadiansystem that is going through a phase adjustment toa new schedule. Also, the shorter rotation does notallow a sleep debt to accumulate, since the numberof consecutive days when the worker has to engagein day sleep remains small. The tradeoff is that,while they are on night shifts, workers will be out ofsynchrony with their circadian systems and perform-ance could be affected (56,105). Also, rapid rotationmay have a greater impact on family and socialinteractions because of the continual schedule changes(56).

A slowly rotating schedule has the advantage ofallowing greater time for circadian readjustment. Astudy of 42 shift workers at a surface mine showedthat some measures of adjustment (mood, episodesof awakening during sleep) improved during thesecond week of a 2-week shift rotation (46).However, a slowly rotating schedule can result insleep debt and fatigue due to more consecutiveperiods of day sleep. This problem can be exacer-bated if the worker reverts to a diurnal schedule ondays off.

Since a weekly shift rotation results in bothinsufficient time for the body clock to readjustcompletely and enough time to build up a sizablesleep debt (158), it is this schedule of rotation that ismost likely to produce physiological problems(8,36,140,146). However, a weekly system mayhave the advantage of allowing a worker to engagemore easily in social and family activities (56).

What is the best rotation system to use? Expertopinion is divided. Many favor rapid rotation withvery short (1- or 2-day) rounds of duty on one shiftbefore changing to a different one (3,140). Othersfavor much slower rotation speeds, with 3 weeks ormore at one shift before moving to a differentschedule (36,11 1). One factor that may help deter-mine the most desirable speed of rotation is the taskto be performed. As previously discussed, there issome evidence that different tasks not only havedifferent best times during the circadian cycle, butalso adjust at different rates to a change in routine(56,57). It may be that simple, repetitive tasks,which are performed relatively poorly at night andadjust rather slowly, would be best performed usinga slow shift rotation, which allows circadian syn-chronization to be maintained for longer periods.Tasks involving immediate retention and high mem-ory loads might be performed relatively well duringthe night shift, even in diurnally oriented individuals(107). In these cases, rapidly rotating shifts might bemore suitable: the circadian system retains itsdiurnal orientation, and there is no gradual buildupof sleep loss (158). Further information is needed todetermine whether the speed of shift rotation canhave differential effects on various types of tasks. Ifthe main goal is to” minimize physiological disrup-tion, a weekly rotating shift seems to be the leastdesirable; either a more slowly (36) or more rapidly(83,179) rotating system may be preferred.


Shift Duration

In rotating shift systems, the length of a shift istypically 8 hours, although longer shifts, often 12hours, are sometimes used. The advantage of 12-hour shifts is that the workweek is compressed into3 or 4 days, giving the worker more days off forleisure and recuperation. The disadvantage is thatthe extra 4 hours of work per day could lead toincreased fatigue and decreased alertness on the job.Such effects have been demonstrated in both labora-tory (139) and field studies (134,138). In one fieldstudy, conducted at a nuclear powerplant, it wasshown that workers on a 3- to 4-day, 12-hour shiftsystem reported increased fatigue and performedmore poorly on tests designed to measure alertnessthan did workers on a 5- to 7-day, 8-hour system(138). These effects were most pronounced in the12-hour night shift, where there was an interactiveeffect between fatigue and the low point of thecircadian cycle. Interestingly, despite these decre-ments, examination of performance on job tasks(e.g., filling out log books, occurrence of unusualevents) conducted at the same time as the othertesting indicated no difference between the 8- and12-hour shift systems (96) (figure 5-10).

A comparison of the number of health complaintsreported by workers in the chemical industry inGermany showed no difference between a rapidlyrotating 12-hour and a weekly rotating 8-hour shiftschedule (62). The use of a compressed workweekwith 12-hour days is also becoming more commonin nonrotating shift schedules (see ch. 4). Whilecompressed workweeks are generally well receivedby workers, they raise concerns about fatigueeffects, longer exposure to workplace hazards (e.g.,noise, chemicals, repetitive motion tasks), andcomplications associated with workers holding asecond job (29). Also, workers who have longcommutes may be at greater risk driving on thehighways. Additional studies are needed to fullyevaluate these factors and to determine how theextended workday can affect performance and safety,especially in regard to night shifts, where circadianeffects would also be present. As with speed ofrotation, the type of job to be performed might affectwhether 12-hour shifts are appropriate. Long shiftsmay be more suitable in jobs that do not requireintense concentration, are not monotonous or dan-gerous, and do not involve heavy physical activity(102).

Figure 5-10-Measures of Performance on 8-Hourand 12-Hour Shifts

2.6 iI

1.6

1.0

i

o—0-0-0— x— x— x~,% —o-u—0—0 —-0

0.6

10 [ \ I I \ 1 I 1 1 1 1 I 1 I

8 am 10 am Noon 2 pm 4 pm 6 pm 8 pm 10 pm Mid- 2 am 4 am 8 am 8 am

Time of day

+ 12-hour day O 8-hour swing● 12-hour night ❑ 8-hour nightX 8-hour day

StandardB. Shift Mean error

8-hour, day 0.174 0.0288-hour, swing 0.188 0.0288-hour, night 0.164 0.02212-hour, day 0.194 0.02512-hour, night 0.112 0.014

A) Number of errors, by time of day, on a laboratory measure ofperformance (grammatical reasoning task) for workers on 8-hourand 12-hourshiftschedules. Significant performance decrementsoccurred on the 12-hour shift. B) The average number of errors ofentering data into log books per hour on 8-hour and 12-hour shifts.There were no differences between shifts.SOURCE: P.M. Lewis, D.J. Swaim, and R.R. Rosa, Ewdusttion of the

12-Hour Shift Schedule at the Fast F/ux Test Fad/ity(Riohland,WA: Pacific Northwest Laboratory, 1986).

Extended duty hours almost invariably result inloss of sleep and fatigue. As previously described,extended periods of sustained work can lead toimpaired performance in most psychological tasks.As a result, periods of extended duty hours withoutadequate time to recuperate from fatigue and lostsleep can have detrimental effects on worker per-formance and safety.

Because of their disruptive nature, schedules thatdivide the day into multiple work shifts interspersedwith rest periods-namely, split shifts-are rare.The most common use of split shifts is for standingwatch aboard merchant and military ships, where a4-hours-on, 8-hours-off schedule is often employed.These schedules have been shown to result in


performance decrements (32,142) as a result ofcircadian rhythm disruption and disturbed sleep. Amore detailed discussion of the shipboard schedulesused in the military is presented in chapter 9.

Sleeping and Napping Strategies

One of the major problems facing shift workers isloss of sleep and the resulting on-the-job sleepiness(5,29). Sleeping and napping strategies designed tooptimize sleep could be useful interventions for theworker. It has been noted that the majority of nightshift workers sleep after work, while day shiftworkers most often sleep before work (151). It hasbeen hypothesized that delaying sleep after workinga night shift is preferable, since such a schedulewould maintain a normal work-off-sleep sequenceand sleep would occur during a period in thecircadian cycle more conducive to longer sleep (45).However, there is little empirical evidence availableregarding this strategy. One survey of 328 workersat a surface mine and two powerplants found thatthose who delayed their sleep after a night shift wereless sleepy than those who slept earlier, but theywere also more likely to report health complaints(45). Since this was a survey study, it could not bedetermined if there was a causal relationship be-tween the sleep schedule employed and the inci-dence of health complaints.

Napping strategies may be useful in two ways.Brief naps taken during temporary periods ofextended work may enhance subsequent per-formance, and naps taken during time off mayhelp to offset the sleep debt typically associatedwith shift work. With respect to on-duty napping, itis known that a sleep period of 1 to 4 hours can lessensleepiness during a 24-hour period of continuousactivity (44,91). One study has shown that airlinepilots who were allowed to nap for a short timeduring a slack period in a flight were more alert whentheir workloads increased (during landings) thanpilots who did not nap (68). Other studies haveshown that naps taken early in a period of extendedwork (42,44) may be more effective in preventingsleepiness than naps taken later during the workperiod are in recovering from sleepiness (133). Inprolonged work schedules (over 24 hours), measuresof performance were improved following prophylac-tic naps, although performance did not return tonormal levels (42,44). This indicates the usefulnessof prophylactic napping during temporary periods ofextended work. The placement of the naps in relation

to the circadian cycle was not a factor in the abilityof the naps to produce their effects. Also, theenhanced performance occurred only after the resid-ual effects of sleep had worn off: performancedecrements can persist for up to an hour afterawakening (9) and may be a limiting factor in the useof naps in work settings where a worker must befully alert on awakening.

Napping at appropriate times during time off mayalso enhance performance by offsetting the sleepdebt that typically accompanies rotating shift work.Recent findings indicate that napping may be anintegral component of the circadian sleep-wakecycle (22,25,26,41). Workers who cannot sleep formore than 4 to 5 hours during their initial attempt (acommon complaint among shift workers trying tosleep during the day) (60,172) may be able to obtaina couple of additional hours if sleep is attemptedagain later in the day (9). It has been suggested thata rotating shift worker should not take a nap if thenext major sleep period after a work shift is to be atnight. A nap under these conditions could interferewith that night’s sleep. If the next night is going tobe another period of work, however, a nap maybebeneficial (9). Also, a nap taken regularly at the sametime of day could act as an anchor, or synchronizingpoint, for circadian rhythms and might slow theadaptation of the rhythms to a new schedule. Thiscould be advantageous to a worker on a rapidlyrotating schedule but detrimental to one on slowlyrotating shifts (135). Thus, naps can be beneficialand provide a period of additional sleep for shiftworkers under some conditions; however, nappingduring the day ultimately interferes with the abilityof permanent night workers to get an adequate,extended period of sleep (154).

Light

As discussed in chapter 3, laboratory studies haveconfirmed that bright light (i.e., light that is equiva-lent to outdoor light at dawn) can act as a resyn-chronizing agent for the human circadian system(34,98,177). This has led to the investigation ofwhether exposure to light can facilitate the circadianadjustment of an individual to shift work (box 5-D).Rigorous use of goggles, lightproof bedrooms, andbanks of bright lights has in some cases entrainedsubjects to non-24-hour routines (50,52).

A recent study of simulated shift work has shownthat a fixed schedule of exposure to bright light


during night work, combined with complete dark- (body temperature, cognitive performance, urineness during daytime sleep, enhanced circadian production) and psychological (subjective alertness,readjustment to night work, compared to exposure to cognitive performance) measures. While it has beennormal room light at night and unrestricted sleep noted that the design of this study did not precludeconditions during the day (35) (figure 5-1 1). The other factors, such as activity and sleep-wakeadjustment was observed in both physiological schedules, from contributing to the circadian re-


Figure 5-1 l—Effects of Bright Light on the Adjustment of One Worker to Shift Work

.37.5

37.0 P

J

37.5 1

36.5 1I

A. Control study

/v37.5 1

First 37.0nightshift

36.5

Standardroom lightduring nightwork

37.5

Sixth 37.0

Lnightshift 36.5

—4 pm Mid- 8 am 4 pm Mid- 8 am

night night

Time of day

r“B. Treatment study

El

Firstnightshift

\

Bright lightduring night workand darknessduring day sleep

Sixthnightshift

1

4 pm Mid- 8 am 4 pm Mid- 8 amnight night

Time of day

The body temperature of a worker on the first and sixth nights of work A) with exposure to normal levels of light and B) with exposure tobright light during the second to fifth night of work. Exposure to bright light led to a shifting of the body temperature cycle to coincide withthe night work schedule (midnight to 8 a.m.).SOURCE: Adapted from C.A. Czeisler, M.P. Johnson, J.F. Duffy, et al., “Exposure to Bright Light and Darkness To Treat Physiologic Maiadaptation to Night

WorlG” New England Journal of Medicine 322:1253-1259, 1990.

adjustment (24), the results do indicate a role forexposure to light and darkness in circadian adapta-tion to night work. It is likely that exposure to lightand darkness as well as activity levels can interact toaffect circadian readjustment (164). It has beenproposed that circadian adjustment to shift workmight be facilitated by manipulating both sleepschedules and exposure to light (5 1). Another studyof exposure to bright light during the night andcomplete darkness during day sleep showed that thetiming of the exposure to bright light determined thedirection of the phase shift in circadian rhythms(52,53).

Exposure to bright light may also enhance certaintypes of performance (23,61) during rotating shiftwork or extended duty hours. Thus, there is someinitial evidence for the usefulness of bright lightto enhance adjustment to work schedules; how-

ever, practical application in the workplace mustawait additional research.

Medication and Drug Therapies

Drugs might be helpful in counteracting the sleepand fatigue problems common to shift work. Onepossible intervention, then, would be to improvesleep by the use of hypnotic medication; however,there are two major problems associated with the useof hypnotics.

The first problem is that of residual effects, whichleave a person drowsy after awakening. Theseeffects (which are characteristic of long-actinghypnotics) can affect the person’s performance andcould therefore cause problems in shift work. Thedevelopment of the short-acting benzodiazepinessuch as triazolam and brotizolam, which have a


half-life of only a few hours (145), has to someextent overcome these problems. It has been shownthat triazolam significantly increases the duration ofday sleep, although this increase did not appear toincrease phase adjustment of the circadian clock,which would have decreased the tendency to fallasleep in the early morning hours (169). Thistendency was unaffected by triazolam. Althoughtriazolam will improve daytime sleep for individualson night shifts, it does little to improve alertness atnight (associated with improved daytime sleep) andnothing to improve nighttime performance (170,171).

The second problem regarding the use of hypnot-ics by workers relates to the chronic nature of thesleep problems associated with shift work. Nohypnotic drugs are recommended for anything otherthan occasional insomnia. The dual problems oftolerance and dependence make it likely that work-ers would derive little or no benefit from anythingother than intermittent use of short-acting benzodi-azepines. Moreover, long-term high doses of tria-zolam have in some cases been associated withamnesia and mood changes (l), effects to which shiftworkers are especially vulnerable. Thus, for ahypnotic drug to provide any lasting benefit, itwould have to be totally benign when administeredchronically. In specialized situations-for example,in the military-occasional use of hypnotics can behelpful in inducing sleep during a period of extendedduty (e.g., to induce sleep during rest breaks foraviation crews exposed to extended periods ofcombat) (91). Residual effects are still a concern,however.

An alternative to more potent hypnotics may bemild over-the-counter sedatives, such as antihis-tamines. These drugs are used by millions ofAmericans and may aid individuals who havedifficulty sleeping (30,92), though these drugs dohave side effects which could limit long-term use.

An essential dietary amino acid, tryptophan, hasbeen shown to be helpful in promoting the onset ofsleep when administered in sufficient quantities(73,74) without causing a significant impairment inperformance (99). Tryptophan is thought to “set thestage for more rapid sleep onset” (147) rather thandirectly inducing sleep. While tryptophan is anaturally occurring substance and is considered safe,the chronic nature of the sleep problems associatedwith shift work could limit its use. Whether theintake of a high dose of a single amino acid over an

extended period can still be regarded as natural isdebatable and has not been tested. Recently, therehave been reports of increased incidence of a rareblood disorder associated with the use of tryptophan.This side effect has been traced to the presence of acontaminant that was inadvertently introduced dur-ing the manufacture of the compound and not totryptophan itself. Currently, the sale of tryptophan inthe United States is prohibited until its safety can beconfined.

The regular use of stimulant drugs to maintainalertness (as opposed to maintaining or inducingsleep) in shift work also raises concerns. While thereis some evidence that the use of caffeine can increasevigilance (99) and may help to increase alertness andperformance on the night shift (18,168), its use canhave detrimental effects on sleep and may causeanxiety, nervousness, and gastrointestinal discom-fort in high doses. As with hypnotics, the occasional,controlled use of more powerful stimulants (e.g.,methamphetamine, dextroamphetamine) has beenstudied to meet the demands of extended duty inmilitary situations (39,91). But concerns about thepossible abuse of these drugs and about the sideeffects associated with them, such as irritability,distorted perception, apprehension, rebound effects(e.g., depression, fatigue), and psychosis after long-term exposure, make their general use inappropriate.Further study is needed to determine the efficacy ofshort-term use of stimulants such as caffeine in shiftwork, especially in occupations in which optimalperformance is necessary (e.g., pilots). Any potentialbenefits would have to be weighed against theproblems associated with use.

As described in chapter 3, the ability to affectbiological rhythms directly and facilitate the reset-ting of the body clock with drugs is being explored.Such drugs could be used to help the circadiansystem adjust to changes in work schedules. Bothmelatonin and triazolam have been studied for theirability to affect biological rhythms. While neitherhas been conclusively shown to have any such actionin humans, there is initial evidence that melatoninhas an effect on the circadian system (97,143). In thecase of triazolam, the circadian effects that havebeen observed in experiments with hamsters (161,162)may be an indirect result of the effects the drug hason activity (114).

Currently, no effective pharmacological agentsare available to assist workers in adapting to shift


work. Long-term use of drugs to counteract fatigueand to ease sleep problems is probably not desirable,and to date no drugs have been conclusively shownto have an effect on the circadian system.

Monitoring

Another possible intervention is the use ofmonitoring devices and instruments to detectindividuals who may pose a risk to themselves orothers because their performance is impaired.A number of tests have been designed to detectdecrements in performance. Some are being used inthe workplace to identify individuals who areimpaired due to substance abuse, but many aresensitive to other causes of decreased performance.The Automated Performance Test System is acomputerized battery of tests that measures alertnessand cognitive and psychomotor factors related to jobperformance. The system can detect impairmentcaused by sleep loss, drugs, and alcohol, but it doesnot indicate the cause of the impairment (77,78,124).

Another computerized test batter-y is the WalterReed Performance Assessment Battery (157). Thisbattery is designed to examine the effects of sleepdeprivation, sustained performance, heat stress,physical fatigue, and physical conditioning in work-ers. Subjects are asked to undertake a variety oftasks, including ones that measure choice reactiontime, time estimation, visual research, pattern recog-nition, sustained attention, short-term memory, men-tal arithmetic, and logical reasoning.

Other instruments to measure performance aregeared more directly toward determiningg the effectsof fatigue and sleep loss (43,136). The NationalInstitute for Occupational Safety and Health, of theCenters for Disease Control in the Department ofHealth and Human Services, has developed aFatigue Test Battery and a Daily Sleep and HabitsQuestionnaire. Included in the computerized FatigueTest Battery are a number of brief performance tasksdesigned to evaluate a range of psychologicalfunctions, including cognitive abilities, perceptual-motor functions, motor skills, and sensory acuity(136-139). The Daily Sleep and Habits Question-naire contains information on sleep and otherpersonal factors known to be affectedly work hours.Some questions relate to the times of retiring andarising (including nap times), sleep latency, numberof awakenings, depth of sleep, and quality of sleepfor the 24 hours preceding the work shift. The

workers are also asked to give their subjectiveevaluation of psychological stress and of gastroin-testinal conditions, to report on personal scheduleadjustments attributable to shift, adjustments ofmealtimes, and exercise periods (138).

These testing instruments are designed to screenindividuals to determine if their performance on atask is suboptimal. Other types of monitoringdevices are designed to measure and record physio-logical and performance parameters as an individualactually goes about performing his or her job. Thesemonitoring instruments are valuable for the collec-tion of research and medical data (104). In addition,they provide real-time, on-the-job feedback to theworker, alert the worker to any decrements inperformance that may occur, and enable the workerto respond appropriately. Such systems could beparticularly useful in job settings where reduction inperformance might have negative safety conse-quences. An example would be a detector thatindividuals could wear to alert them if they began tofall asleep (117). While such a device would notreverse the person’s sleepiness, the feedback itwould provide could be crucial in certain settings(e.g., truck drivers, railroad engineers). The designof workplace systems to provide workers withinformation to judge their performance and torespond to anomalous events would also be valuable(66).

While all of these monitoring procedures might beuseful, it has been noted that on-the-job monitoringitself can induce stress (163). Such stress couldcompound the stress associated with work sched-ules. Additional research is needed to developmonitoring instruments and systems and to deter-mine the role they could play in various job settings.

Employee Education and Clinical Support

Given the inherent nature of the stress causedby circadian rhythm disruption and the otherfactors associated with shift work, an importantintervention is educational programs that pro-vide workers with strategies to deal with theproblems they face and to ease some of theconsequences (109). Information such as the besttiming of sleep, meals, and other activities tocoincide with a given schedule could be helpful.Guidance on ways to make the home more condu-cive to daytime sleep and the need for cooperationamong family members to ensure that the worker is


able to get as much sleep as possible would also bebeneficial. Finally, providing the entire family withsupport and guidance that will help them deal withproblems that occur could result in a more support-ive domestic environment for the worker. Furtherresearch is needed to determine what types ofeducational programs are most effective in helpingworkers cope with their schedules.

Support for shift workers is largely lacking in U.S.workplaces. Although sleep disorder clinics areavailable, most of these facilities are not designed todiagnose and handle the underlying problems thatcause sleep and other related disorders arising fromwork schedules.

Fitness

The possibility that improved physical condition-ing can increase the rate of adjustment to shift workor increase an individual’s tolerance to it has beenexamined in a few studies (70,71). These studieswere conducted on 75 female nurses engaged in shiftwork and sought to determine whether improvinglevels of overall fitness would facilitate their abilityto cope with shift work. The 4-month exercisetraining program that was implemented elevatedrespiratory efficiency by 7 percent. Improvementswere noted in self-reports of sleep length, night shiftalertness, and general fatigue. Another study failedto show any effect of physical conditioning on theadaptation of wake-sleep and body temperaturerhythms to a change in work schedule (135). Thus,the data collected so far suggest that the benefits ofphysical conditioning may be related to an increasein general strength and more positive subjectivereports rather than to any direct facilitation ofadjustment to shift work. While these studiesindicate that physical fitness might be helpful,additional research is needed to confirm these resultsand to delineate clearly what mechanisms producethe observed effects. It has also been noted that whilephysical fitness may lessen the effects of physicaland mental fatigue, it cannot protect against theeffects of sleep loss (16).

RESEARCH NEEDSSubstantial progress has been made in under-

standing the underlying physiological mechanismsinvolved in the generation, entrainment, and expres-sion of circadian rhythms in animals over the lasttwo decades. Similar advances in the application ofthese data to workplace issues have not been made

or supported in this country. A principal need is forresearch into how to apply basic knowledge oncircadian rhythms and sleep to field studies in theworkplace. Since demands differ from oneworkplace to another, research is needed to establishcommon chronobiological principles, as well astask-specific applications, that can be transferredfrom the laboratory to the workplace. While someresearch programs, notably those of the Departmentof Defense and NASA, use this approach, it isnecessary to apply it also to civilian shift work—andnot only in industrial settings, but in other settings aswell (e.g., police work, long-distance transportation,and medical care). Physiological parameters, such asbrain activity, temperature, and endocrine levels,need to be monitored and associated with psycho-logical states and performance measures in order tobetter understand how functioning is perturbed andhow perturbations could be ameliorated. Whilelaboratory studies can attempt to mimic and isolaterelevant variables, they cannot fully identify orsimulate all the factors present in natural situations.

Research designs need to incorporate enoughpeople over enough time to obtain statistically andbiologically meaningful data. Furthermore, analyticmodels similar to those developed for operationsresearch are needed to take into account the multipleinput and output variables. The traditional experi-mental-control design may not be adequate for manyof the research questions that need to be addressed.

Research is needed on the variables that contrib-ute to difficulties in adjusting to shift work. Factorssuch as individual differences in circadian rhythms,sleep patterns, personality, and age may play a rolein the ability of an individual to cope with a workschedule. Studies of the roles of familial andnonwork social demands on the shift worker are alsoneeded. Living in a society that has a differenttimetable can impose extra burdens on the worker,producing additional stresses on his or her health andwell-being.

Sleep loss and fatigue induced by shift work aredependent not only on duration of work, but also ontime of day and time of cycle. These parameters needto be documented, and their roles in the ability ofworkers to perform safely and effectively need to beestablished. The complex effects of biologicalrhythms, health status, and drug and alcohol use onperformance of workers exposed to the additionalphysiological stresses of disturbed sleep and cir-

Chapter 5--Biological Rhythms and Work Schedules .113

cadian rhythms have largely been ignored by re-searchers.

In addition, there is a compelling need for moreresearch on the interaction between work schedulesand safety in the workplace. Central to this endeavoris more thorough collection of workplace dataregarding hours of work and the occurrence ofon-the-job accidents. Increased data collection byrelevant Federal agencies would greatly facilitatethis process.

All of the previously stated areas of research willcontribute to the final area of research activity-thedevelopment of appropriate intervention techniquesor countermeasures to promote coping and improveadaptation to shift work. Linked to this is the needfor adequate surveillance of workplace performanceand evaluation of the effectiveness of potentialinterventions.

SUMMARY AND CONCLUSIONSThe disruption of biological rhythms that occurs

as a result of nonstandard work schedules caninteract with other factors to degrade the sleep,health, performance, and well-being of individuals.Although the degree to which different scheduleshave these effects, which types of tasks are affectedmost, what variables make an individual susceptibleto them, and what interventions can be taken tolessen the effects have not been clearly delineated,some general conclusions can be drawn.

The stresses associated with disruption of cir-cadian cycles, sleep deprivation, and social dishar-mony interact and affect a number of areas. Shiftwork is associated with increased incidence ofgastrointestinal problems, may be a risk factor forother health problems, and for some workers resultsin a general, chronic malaise. Performance on sometypes of tasks can be diminished, especially in thosesettings where sleep loss and fatigue compound theeffects of circadian disruption. These performancedecrements can compromise the safety of the workerand can threaten public safety as well.

While the most direct intervention is to deviseschedules that do not lead to these consequences, thedevelopment of a single best type of schedule is notpossible. The demands of a given job will helpdictate what might be best in that situation. Theavoidance of weekly rotating schedules maybe mostpreferable from a circadian perspective, and sched-

ules that incorporate extended duty hours whichresult in sleep loss and fatigue should be avoided.Beyond managing schedules, implementing educa-tional and support programs regarding biologicalrhythms, sleep, and family counseling may be agood way to improve the coping ability of shiftworkers. The development and use of such mecha-nisms as bright light and drugs to help the circadiansystem desynchronize and napping strategies tocounteract the effects of sleep loss and fatigue wouldalso be beneficial to workers. Finally, the develop-ment of monitoring devices and systems that couldbe used to assess performance capabilities could alsoplay a role in intervention.

Thus, while general information is available onbiological rhythms and shift work, specific informa-tion is lacking. There area number of areas in whichthere are gaps in the base of knowledge. Theinformation derived from additional research willdefine the nature, characteristics, and magnitude ofthe effects caused by work schedules. This knowl-edge will guide the design and implementation ofintervention measures, which are necessary to re-duce job-related stress, health effects, and fatigueand to improve alertness and safety.

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Biological Rhythms: Implications for the Worker (Part 7 of 19)