Animal sleep patterns lessmore 4hrs20hrs16hrs3hrs8hrs Sleep patterns vary by species psychlotron.org.uk

Table 1. Representative Total Sleep Requirements for Various Species

Comparative studies of different species generally state: 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 WHY? psychlotron.org.uk

Evolutionary Theory - we evolved to sleep so that we would conserve energy when we were least efficient - predicts that species will sleep different amounts depending on how much they must look for food and watch for predators

Evolutionary Explanation of Sleep

Examples: Energy Conservation Sleep uses less energy than wakefulness Small animals with high metabolic rates need to conserve their energy more Mouse sleep time: c. 12hrs

Foraging Requirements Herbivores spend their time eating plants, which are relatively low in nutrients They cannot afford to spend too much time sleeping Horse sleep time: c. 2 hrs

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

Predator Avoidance If an animal is a predator, it can sleep longer than prey species because it has less predation risk Sleep should be when an animal is least vulnerable Lions: c hrs

However: Waste of Time Theory Meddis (1975) sleep helps animals stay out of the way of predators Most animal should sleep during darkness Siegel (2008) concurs. Sleep enables both energy conservation and staying out of danger Little brown bat: Awake when it needs to be?

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

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 that 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  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?

Lets have a debate!!! Restoration v Evolutionary 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 can 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 begin this towards the end of our revision class today