Theories of sleep

Hibernation theory (Webb, 1974) Evolutionary approach – sleep is

assumed to confer a survival advantage Sleep helps conserve energy by:

Reducing energy spent on behavioural activity

Drop in body temp & metabolic rate reduces energy lost as heat

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Hibernation theory

Main prediction is that an animal’s sleep pattern will be determined by its energy intake & expenditure pattern Small animals will need more than large Diet (e.g. carnivore vs. herbivore) will affect

sleep patterns Sleep will increase when energy loss goes

up (cold) or energy intake goes down (food scarcity)

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Animal sleep patterns

less more

4hrs 20hrs16hrs3hrs 8hrs

Sleep patterns vary by species

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Evidence

Comparative studies of different species generally support evolutionary view Smaller animals tend to sleep more than

larger (e.g. giraffe 1hr vs. bat 20hrs) Carnivores sleep more than herbivores (e.g.

lion 16hrs vs. buffalo 3hrs) Some notable exceptions e.g. rabbit

(small, herbivore) & human (much larger, omnivore) both sleep about 8hrs

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Evidence

Sleep patterns are affected by energy expenditure & availability Animals generally sleep more when

weather is cold and food is scarce (Berger & Phillips, 1995)

However, no direct correlation between physical work done and sleep duration in humans (e.g. Horne & Minard, 1985)

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Problems

Many evolutionary significant factors could affect sleep patterns; theory doesn’t tell us which are important Does a bat get so much sleep because it’s

small or because it has few predators? Why do animals with very different

lifestyles have similar sleep patterns?

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Problems

Some features of sleep cannot be explained easily by the hibernation theory: Why is sleep universal when in some

species (e.g. dolphins) it would have been an advantage to get rid of it?

Why is sleep deprivation apparently fatal?

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