Why do we sleep? – Evolutionary Theory

1. 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!! Approaches - Evolutionary , comparative Issues - use of non-human animals, Debates - Reductionism Methodology – problems with the Phylogenic sleep project

2. 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 psychlotron.org.uk

3. 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 psychlotron.org.uk

4. Animal sleep patterns Sleep patterns vary by species 3hrs 4hrs 8hrs 16hrs 20hrs psychlotron.org.uk less more

5. 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 psychlotron.org.uk

6. 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) psychlotron.org.uk

7. 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? psychlotron.org.uk

8. 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? psychlotron.org.uk

9. 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.

10. 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.

11. 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?

12. 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’

13. 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!!

14. 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?

15. 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.