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Why do we sleep? – Evolutionary Theory. BATs AO1 – Understand and recall the ecological theory of sleep A02 – Analyse information and relate it to psychological theories - Use information from psychological research to evaluate the theories. Good old synoptic points!!.

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why do we sleep evolutionary theory
Why do we sleep? – Evolutionary Theory


AO1 – Understand and recall the ecological theory of sleep

A02 – Analyse information and relate it to psychological theories

- Use information from psychological research to evaluate the theories

Good old synoptic points!!

Approaches - Evolutionary , comparative

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

Methodology – problems with the Phylogenic sleep project

theories of sleep
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


hibernation theory
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


animal sleep patterns
Animal sleep patterns

Sleep patterns vary by species









  • 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


  • 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)


  • 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?


  • 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?


evaluation of evolutionary explanations
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 explanations1
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
Waste of Time Theory – Meddis1975
  • Read p 14 – Waste of time and Cutting Edge and p15 – Unilateral sleep
  • What did Meddis propose?
  • Why did Siegel and Young (2008) support Meddis.
  • What other explanation does their idea have for why bats sleep so little?
  • What methodological issues are there with the Phylogeny of Sleep project?
  • Why have some species adapted Unilateral sleep?
a summary of the hibernation ecological theory of sleep
A summary of the Hibernation/Ecological Theory of Sleep
  • Compare the ideas of Webb (1982) and Meddis (1975) – worksheet matching activity
  • What evidence is there to support the predictions outlined on the worksheet (Ecological Theories of Sleep)?
  • 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
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
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?
  • 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.