Outline and evaluate Evolutionary theory

The hibernation theory of sleep (Webb, 1982) proposes that the function of sleep is to conserve energy, as it provides a period of enforced activity, much like hibernation. The reason for this is that mammals, such as ourselves, expend a lot of energy in order to maintain a constant body temperature (a homeostatic mechanism). Animals with high metabolic rates, such as mice, expend even more energy. Energy conservation is also influenced by foraging requirements. Animals such as herbivores that eat food with little nutrients, need to spend more eating to promote survival, and therefore have less time to sleep. Conversely, carnivores who eat food high in nutrients, spend less time eating and can therefore spend more time sleeping to conserve energy, promoting survival. 

Research conducted by Zepelin and Rechtschaffen (1974) supports the hibernation theory. They found that smaller animals with higher metabolic rates, sleep more than larger animals. This supports the idea that sleep enables energy conservation when much energy is expended. 

 Conversely, Cappelini et al. (2008) found a negative correlation between metabolic rate and sleep, whereby smaller animals with higher metabolic rates sleep less. This suggests that the hibernation theory may be invalid and indicates that sleep may serve an alternative function. 

It is essential to consider a fundamental issue with correlational research, whereby we cannot infer causation. It would be difficult to conclude for example, whether less sleep amongst herbivores is due to the fact that they need to forage for longer, or whether this is due to the fact that they are prey animals. Thus, conclusions about the functions of sleep and their adaptive nature, may lack validity.

Additionally, whilst Cappelini et al. (2008) opposed the hibernation theory, they did find that greater foraging requirements create a restraint on time available for sleeping. This supports the view that sleep is influenced by foraging requirements due to a diet lacking in nutrients or higher metabolic rate.

When considering the function of sleep, it is important to distinguish between NREM and REM sleep. Allison and Ciccetti (1976) found that larger animals had less NREM sleep but not less REM sleep. This supports the view that only NREM has evolved for energy consumption. 

Another evolutionary explanation is predator-prey status. If an animal is a predator it can sleep for longer, whereas prey sleep less in order to remain vigilant. Meddis (1975) proposed the ‘waste of time’ hypothesis, suggesting sleep helps animals to stay out of danger when they are most vulnerable and when they have nothing else to do. Diurnal animals cannot forage for food at night and therefore stay out of harms way. 

Support for this explanation was provided by Alison and Cicchetti (1976). They reported that species who had a higher risk or predation did sleep less, although there were exceptions such as rabbits, who slept as much as some animals in less danger. This suggests that sleep may indeed be influenced by the need to remain safe and avoid predators.

However, one could argue that sleeping to avoid predators would actually leave animals unreactive and vulnerable. Instead, if safety was the only consideration, it would be more effective to stay awake and alert. 

There are a number of issues with the fact that the evolutionary approach relies heavily on studies involving non-human animals. Observing animals in the wild may be unethical, if this disturbs their natural behaviours. Moreover, observations in captivity may lack ecological validity, as animals may behave differently to how they would in the wild. Sloths have been reported to sleep far longer in a zoo than in the wild, which may be due to a lack of concern over predators and no need to forage for food. 

A problem with evolutionary explanations, is the failure to account for the fact we possess such a strong desire for sleep, when sleep deprived. Horne (1988) proposes a theory that combines elements from both restorative and adaptive theories. He suggested core sleep (SWS) is essential for body and brain processes, whereas optional sleep (REM and some portions of NREM) serve the purpose of occupying unproductive hours and conserving energy. 

Nevertheless, evolutionary explanations do acknowledge the complexity of behaviour by considering a range of variables such as the ecological niche of the animal (habitat, whether predator prey and whether nocturnal or diurnal), as well as considering EEG recordings and physiological measurements. One could argue therefore that unlike many other psychological explanations, this adopts a holistic approach and is not reductionist.