Discuss evolutionary explanations of the functions of sleep. Evolutionary explanation of sleep suggests there four mains reasons for it, energy conservation, foraging requirements, predator avoidance and to waste time. 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. Energy conservation is the theory that warm blooded animals need to expend a lot of energy to maintain a constant body temperature, their high metabolic rates therefore cause them to need more sleep as a way of providing a period of enforced inactivity Webb 1982 described this as hibernation theory. Zepelin and Rechtschaffen (1974) found that smaller animals, with higher metabolic rates, sleep more than larger animals. This supports the view that energy conservation might be the main reason for sleep. However there are many exceptions, such as sloths, which are very large yet sleep for 20 hours a day. Another evolutionary explanation of the function of sleep is foraging requirements.
The time spent sleeping may be constrained by food requirements, herbivores have constrained sleep as they have to eat grass constantly due to lack of nutrients such as cows, whereas carnivores however can afford to rest much of the time as they eat a variety of meats giving them these important nutrients. Predator avoidance is another evolutionary explanation which suggests that if an animal is a predator then it can sleep for longer, whereas prey species cannot afford to sleep long as of predator risk. If sleep is a vital function then they are best to sleep when east vulnerable. Allison and Cicchetti (1976) found that species who had a higher risk of predation did sleep less, though again there were exceptions, such as rabbits who had a very high danger rating yet slept as much as moles who had a low danger rating.
Finally Meddis (1975) suggested the theory of ‘waste of time’ hypothesis. He suggested that sleep helps animals to stay out of the way predators during the parts of the day when they are most vulnerable. For most animals, this means sleeping during the hours of darkness, it also means sleeping in places where they will be hidden. Siegel (in Young, 2008) concurs with Meddis’s view and points out that in fact, being awake is riskier than sleeping because an animal is more likely to be injured. The main function of sleep is that it enables both energy conservation and keeping an individual out of danger: ‘in the wild, the best strategy for passing on your genes is to be asleep for as long as you can get away with … and that is exactly what you see’ (Young, 2008). Because of the weaknesses in these theories 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.
Capellini and her research team (2008) argued that previous research was flawed because the methods used to collect data on sleep in different animals were not standardized and therefore comparisons between species were meaningless. They carefully selected data from studies using only standardized procedures (e.g. animals habituated to lab conditions). The study focused on only land mammals because unilateral sleep in aquatic mammals involves different sleep patterns. They found a negative relationship between bodies mass and sleep (i.e. smaller animals slept more) which doesn’t support the energy conservation hypothesis. However this data supports the view that there is a trade-off between sleep and foraging – Greater foraging requirements create a restraint on time available for sleeping.