Why do we sleep? – Evolutionary TheoryBATsAO1 – Understand and recall the

ecological theory of sleepA02 – Analyse information and relate it to

psychological theories - Use information from psychological

research to evaluate the theoriesApproaches - Evolutionary , comparative

Issues - use of non-human animals, Debates - Reductionism

Methodology – problems with the Phylogenic sleep project

Good old synoptic points!!

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)

Read and complete the worksheet – Ecological Theories of Sleep

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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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Evaluation of Evolutionary explanations

A criticism of all evolutionary explanations is that arguments about adaptiveness and natural selection are made post hoc (‘after the fact’ – after it happened).

We assume the sleep (and sexual selection!) is adaptive because they are still around today.

But they may just be neutral and not selected out or linked to another adaptive behaviour.

Evaluation of Evolutionary explanations

The ecological (evolutionary) theory of sleep is the same as Circadian theory!

Circadian rhythms evolved to induce sleep at appropriate times of day – so they are linked.

Waste of Time Theory – Meddis1975

Read p 14 – Waste of time and Cutting Edge and p15 – Unilateral sleep

1. What did Meddis propose?2. Why did Siegel and Young (2008)

support Meddis.3. What other explanation does their idea

have for why bats sleep so little?4. What methodological issues are there

with the Phylogeny of Sleep project?5. Why have some species adapted

Unilateral sleep?

A summary of the Hibernation/Ecological Theory of Sleep

1. Compare the ideas of Webb (1982) and Meddis (1975) – worksheet matching activity

2. What evidence is there to support the predictions outlined on the worksheet (Ecological Theories of Sleep)?

3. What evidence is there that challenges these ideas?

You can also use p 14-15 and the Worksheet ‘Theories of sleep – Hibernation theory’

Lets have a debate!!!

Which do you think is the most plausible theory for why we sleep?

Jot down the pros and cons of each theory

Choose either the Restoration or Evolutionary Theory.

Share all the good points you van think of with others who have chosen the same Theory

Be prepared to speak in a debate!!

Extension Activity

Read Horne’s interview and page 15 – A combined approach

Why might the Restoration theory and Evolutionary theory be thought of as Reductionist?

How does Horne’s theory address this problem?

Homework

Discuss evolutionary explanations of the function of sleep. (9+16 marks)

You can use the Restoration theory for A02 material as long as it is relevant.