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Sleep and Biological Rhythms

6) Functions of Sleep – Evolutionary Explanations

The evolutionary explanation of sleep tries to explain why we sleep through the benefits it would bring in terms of survival.

Energy conservation

Mammals expend a lot of energy just staying awake at a certain body temperature. We have a limited supply of energy so sleep acts as a way of reducing the amount of energy we use up each day as it is a period of ‘enforced inactivity’. Also, this can explain why some animals sleep more than others; smaller animals have a greater need for saving energy as their metabolism is higher, hence why they sleep more. Webb called this the ‘hibernation theory’ of sleep.

Foraging requirements

This explanation can help us understand why some animals spend more time sleeping than others. Herbivores eat food which is low in nutrients, so in order to get the nutrients their bodies’ need they need to spend more time eating, and therefore spend less time sleeping. This could explain why some animals sleep much longer than others.

Predator avoidance

‘Predator avoidance’ is another way in which we can understand why some animals sleep more than others. If an animal is a predator it doesn’t have to worry about being attacked while it sleep, so it can sleep for long periods of time, conversely prey must spend as much time as they can afford to awake in order to avoid predators.

‘Waste of time’

According to Meddis, sleep ensures that animals stay still and out of the way of predators when they have nothing better to do, so sleep is simply a way of wasting time. Seigel agrees with this view, saying that actually being awake and moving is more dangerous for prey as the animal is more likely to get injured.


Support from statistics – If we look at the sleeping patterns across the animal world they, in general, fit with the evolutionary explanations. For example, smaller mammals sleep longer (Zepelin and Rechtschaffen), which fits with the energy conservation theory. Similarly, Alison and Cicchetti found that animals who had a higher risk of predation did sleep less, which fits with the predator avoidance explanation.

Why do we crave sleep more when we are sleep deprived? – This explanation fails to answer some key questions about the function of sleep. For example, why do we have a stronger drive for sleep when we are deprived of it?

A combined approach

Because of these weaknesses many psychologists have suggested a ‘combined approach’. Horne (1988) proposed a theory which combined both the restorative and evolutionary theories to explain the functions of sleep. He said there are two types of sleep, core (which is needed) and optional sleep. Horne proposed that the restorative approach can explain why we need core sleep, whilst the optional sleep can explain why we might sleep to waste time and conserve energy.


About Sam Cook

A blog set up to help A Level students revise Sociology


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