The basis of the evolutionary theory is that animals sleep because it serves some adaptive function. The notion of the adaptive nature of sleep comes from the theory of evolution. The idea is that any behaviour that has continued into an animal’s gene pool must have been naturally selected because it has, in some way, aided the survival and reproduction of the animal that possesses that characteristic.
One explanation of sleep that the evolutionary theory proposes is that it helps to conserve energy, particularly in warm blooded animals such as mammals which use a considerable amount of energy to maintain their body temperature - This especially true from animals with high metabolic rates such as mice. According to the theory, sleep provides a period of enforced inactivity to conserve energy which Webb explained as the Hibernation Theory of sleep. It is important when thinking about the function of sleep to distinguish between the two different types of sleep, core sleep and other types of sleep. The two main types of core sleep are slow wave sleep (SWS) and REM sleep. It is possible that they both serve a different purpose. Research has shown that energy is only conserved in NREM sleep because the brain is relatively active in REM sleep which suggests that only NREM sleep has evolved for energy conservation. More primitive animals such as reptiles only have NREM sleep, supporting the idea that NREM sleep evolved first for energy conservation and REM sleep evolved later to maintain brain activity. This is supported Allison and Cicchetti who found that larger animals had less NREM sleep but not less REM sleep which shows NREM sleep is important for energy conservation. However, this is not supported by Capellini who found no correlation between body size and sleep.
If the evolutionary theory’s ideas about the relationship between energy conservation and sleep are correct then we would expect small animals with higher metabolic rates to sleep more than larger animals. The theory predicts a negative correlation between body size and sleep which is supported by Zepelin and Rechtschaffen, who found smaller animals sleep more than larger animals, although there were some exceptions such as sloths that are large and sleep a lot. However, this research is not supported by Capelini et al who found a positive correlation. The study used standardised data and had a more robust design as it was a laboratory experiment with controlled conditions and so may therefore be more reliable than the older evidence.
Another explanation the theory proposes is the idea that if sleep is necessary then the time spent asleep conserving energy is constrained by foraging requirements. The foraging requirements theory predicts a trade-off between foraging needs and sleep. For example, Capellini found that animals with a greater need for foraging because of a high metabolism or diet low in energy had lower sleeping rates, such as herbivores that have to spend a lot of time eating because their food is poor in nutrients, supporting the foraging requirements explanation for sleeping patterns. The evolutionary approach suggests we sleep because it is adaptive otherwise all animals there is a question as to why all animals do it and why their sleep patterns differ. This suggest that these patterns are adaptive to the species surroundings. The phylogenetic scale provides support for the evolutionary nature of sleep as the fact that animals that are close together on the phylogenetic scale share close similarities in their sleeping patterns supports the view that these patterns have evolved through adaption to environmental pressure. The phylogenetic scale can also help to explain the relationship between behaviour and sleep patterns in humans and other species.
Sleep may also provide protection to animals against predators and time spent asleep may be constrained by the risk of predation. For example, predators can sleep for longer as they are less likely to encounter danger whereas prey species are in danger when sleeping, if sleep is vital it must be done when they are least vulnerable such as when they can hide. The evolutionary approach predicts a trade-off between predation risk and sleep which is supported by Allison and Cicchetti who found that animals with high predation risks did sleep longer than those with low predation risks, supporting the idea that sleep has evolved as a predator avoidance mechanism. However, Capellini et al found the relationship is complex, for example animals that sleep socially sleep fewer hours but should sleep more because there is safety in sleeping in numbers. Sigel also pointed out that being awake is dangerous, not just because of the risk of predation, but also because an animal is more likely to be injured. It may not be possible to generalise the findings of animal studies to the affects that sleep has on humans as they are physiologically different and also there may be ethical implications of using animals in psychological research because of the risks it may pose.
There is also the idea that sleep has evolved as a way of wasting time. Meddis proposed that sleep is what animals do when they have nothing else to occupy their time like foraging for food or keeping out of harm’s way. For most animals this means at night and out of sight because they are less likely to be seen by predators and so are less vulnerable. The theory argues that the best adaptive strategy is to sleep as long as possible for example, the little brown bat, a small mammal with a high metabolism, should sleep little. However, it has a very specific diet of insects that are around for a short period of time each day so the bat is only awake then. This illustrates how animals have evolved in order to adapt to their surrounding and different sleep patterns.
The evolutionary theory of sleep has been criticised for being reductionist and explaining the complex mechanisms of sleep as a single facts. For example, some animals go to great lengths to sleep such as dolphins which have evolved so that they can rest different parts of their brain at one time and the question has been raised as to why such as complex mechanism has evolved just to keep them out of harm’s way. There have also been no reports of individuals who have managed to live without sleep which supports the restoration theory’s explanation that sleep hasn’t just developed in order to waste time and is in fact essential for maintaining the function of the brain and body. It may be that in order to understand the function of sleep properly we must view it using a combined approach of both evolutionary theories which may explain where and when we sleep and restoration theory which may explain why.