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

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) psychlotron.org.uk

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

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

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

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

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