The four main evolutionary theories all explain sleep as an adaptive response to aid survival.
Energy conservation theory suggests warm blooded mammals must conserve energy in order to maintain a constant body temperature. This is problematic for smaller mammals as they have higher metabolic rates therefore use more energy. Evolutionary theorists argue sleep enforces periods of inactivity to conserve energy, as metabolism is lower when asleep.
Food requirements have been suggested to determine sleep. All animals need to eat to survive therefore sleep may have evolved around this. Herbivores such as cows eat food low in nutrients therefore spend a vast amount of time grazing, and little time sleeping. Carnivores such as lions eat food high in nutrients therefore can afford to spend less time eating. they also have no predation risk therefore can spend more time sleeping.
Meddis proposed the 'waste of time hypothesis' suggesting sleep serves the purpose of helping animals to avoid predators when they are most vulnerable, as sleeping ensures they remain still when they have nothing better to do.
Predator avoidance links to this and Siegel et al argued it would be riskier for the animal to stay awake due to predator risk and risk on injury. He suggested sleep could enable both energy conservation and predator avoidance, as long as the animal sleeps in hidden safe places.
Cappellini et al found a negative correlation between metabolic rate and sleep in smaller mammals, having a higher metabolic rate but sleeping less supporting that food requirements may influence sleep. Smaller mammal, with a high metabolic rate, will have to eat more frequently in order to sustain their high metabolic rate, creating a restrain on sleep, as they must spend more time foraging. Suggesting a trade off between sleep and foraging. Within the study a standardised procedure was used therefore provides more reliable results with higher internal validity. However, this study only focuses on land mammals therefore may not be generalisable to all species, therefore lacks external validity. It also does not explain how evolutionary pressures affect sleep patterns for all species.
Horne et al proposed a combined approach to try and explain areas which a purely evolutionary theory failed to explain such as why we have a strong drive for sleep when deprived. This approach proposed some elements of sleep are for restoration and some are for occupying unproductive hours e.g. conserving energy, for small animals. This suggests that evolutionary approaches do have validity however individually they cannot fully explain function of sleep. Horne also proposed a distinction between core and optional sleep. Core sleep is equivalent to SWS and optional is equivalent to REM. Optional sleep is argued to be dispensable and used to occupy unproductive hours and used to conserve energy supporting the energy conservation theory. However, restoration theorists would argue REM plays an important role in brain functioning and is not simply used to waste time.
Both Capellini and Horne's research can provide support towards some explanations with the evolutionary approach of sleep. However, research cannot provide conclusive evidence towards one sole evolutionary theory highlighting the subject is inconclusive and debatable. However, they suggest how sleep has evolved to aid survival for example the Indus dolphin sleeps for short periods of time, of a few seconds, in order to avoid debris. Such pressures could be argues to play an overriding factor in sleep behaviour as they determine the animal's survival chances, providing support for an evolutionary function of sleep.
Many studies are based on animals therefore findings cannot be generalised to humans as our environmental pressures are different e.g. we have no predator risk and food is generally available. Therefore according to the evolutionary approach we should not need to sleep, but we sleep for 8 hours a day. This highlights a possible biological and restorative need rather than being controlled by evolutionary pressures. Humans generally dictate their sleep patterns, yet evolutionary explanations ignore this therefore are deterministic, ignoring our own conscious decisions in sleep patterns.
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