Evolutionary Explanations of the Function of Sleep
What is the Evolutionary approach?
• We know that sleep must be adaptive in some way, otherwise why do all animals do it despite substantial costs?
• Either it provides some vital biological function, or it provides some other benefit.
• The Evolutionary explanations aim to suggest what other benefits might be associated with sleep.
• The evolutionary approach has also been called the ecological approach. It is called ‘ecological’ because it is based on observations of animals in their natural environment;
• ‘Ecology’ is the study of animals in relation to their environment.
• Although all animals sleep, patterns vary:
– Nocturnal / Diurnal
– Amounts of sleep
– Amounts of REM / NREM
– So there are different explanations within the evolutionary perspective
EVOLUTIONARY (ECOLOGICAL) EXPLANATIONS
• Energy conservation
• Predator avoidance
• Foraging requirements
• Waste of time
• All activities use energy, and animals with high metabolic rates use even more energy.
• Sleep, however, serves the purpose of providing a period of enforced inactivity (therefore using less energy)
In some animals, hibernation is a means of conserving energy. Webb (1982) described this as the hibernation theory of sleep
• Availability of food: animals sleep more when food is scarce (Berger & Phillips 1995).
• Zepelin and Rechtschaffen (1974) - smaller animals, with higher metabolic rates, sleep more than larger animals.
• If an animal is a predator, then it can sleep for longer whereas prey species sleep time is reduced as they must remain vigilant to avoid predators.
• if sleep is a vital function then they are best to sleep when least vulnerable.
• Shrews in burrows & bats in caves can sleep more safely than antelopes or zebras on the open savannah
• Could sleep be more risky that quiet alertness? Are you more likely to be eaten if you’re asleep?
• But research support (Allison & Cicchetti 1976; Lesku et al 2006)
– predators have more total sleep time than prey