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Outline and evaluate one or more evolutionary explanations for the functions of
(8 marks + 16 marks)
Evolutionary explanations of sleep have an adaptive purpose to maximise the chances of survival and
pass on the genes to offspring.
One of the main features of the evolutionary explanation is sleep conservation, as proposed by
Webb (1982). Mammals use lots of energy to keep body temperature constant, find food and avoid
predators in the wild, especially animals with high metabolic rates such as rodents which use more
Webb suggested that sleep serves the purpose of providing a period of inactivity, therefore
conserving energy which would be more efficiently spent at other times (e.g. during the day when
it's easier to find more food or a partner).
A strength of the evolutionary explanation of the functions of sleep is that it has been supported.
Evidence by Rechstaffen (1974) found a negative correlation between the metabolic rate of animals
and the amount of sleep they have, therefore supporting Webb's theory that animals need to sleep
to avoid depleting all their energy and dying.
However these studies can be criticised as being correlational which do not necessarily provide a
cause and effect between sleep and metabolism, because it doesn't necessarily mean sleep is a
consequence of a high metabolism when it could be the other way around. For example the
three-toed sloth has a metabolism so slow that it can die on an full stomach, though it sleeps for 18
hours a day which suggests that there are other extraneous variables which are not taken into
account, therefore lessening the internal validity of these studies.
Capellini et al (2008) found that smaller animals with higher metabolic rates sleep less than larger
animals with lower metabolic rates, therefore contradicting Webb's theory.
This evidence, however, can still be interpreted as supporting the idea that sleep is adaptive
because it suggests there is a trade-off between sleep and foraging. If sleep is a necessity, the time
spent sleeping may be constrained by food requirements as herbivores such as cows eat plants
relatively low in nutrients therefore don't sleep much to make up eating time, whereas carnivores
such as cats eat food high in nutrients which allows them to sleep more.
Overall, Capellini's study and foraging function of sleep is more likely to be valid because there were
methodological issues present in Rechstaffen's energy conservation study. Rechstaffen's research
was flawed because the methods used to collect data on sleep in different animals were not
standardised and therefore comparisons between species were meaningless; instead Capellini
carefully selected data from studies only using standardised procedures in lab conditions making
research into sleep for foraging requirements more internally valid and credible as an explanation.
Other examples in the animal kingdom support the idea that sleep patterns such as unilateral sleep
in dolphins and birds- may have evolved in response to environmental pressures, as well as sleep
duration. This is an evolutionary explanation of sleep because it helps a marine mammal and migrating
birds to cope with particular pressures to survive.
A dolphin must swim to the surface every time they need to breathe. A dolphin that fell into deep
sleep (SWS) while underwater would drown, so the two hemispheres in the brain swap over every
few hours. Similarly, migrating birds must remain awake for long periods of time so also use unilateral
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sleep. The fact that unilateral sleep has separately evolved in both of these different animals shows
it is a means of solving the evolutionary pressures facing these animals.
There is a further argument for the REM/NREM distinction because not animals have REM. Animals
which are more primitive, such as reptiles, only have NREM sleep. It might be that NREM sleep
evolved first for energy conservation, whereas REM sleep evolved later to maintain brain activity.…read more