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Outline the evolutionary explanation for the function of sleep (8 marks)
The evolutionary explanation aims to suggest what other benefits might be associated with sleep.
The evolutionary theory states that sleep is serving some adaptive function related to survival and
therefore occurring through natural selection. Different species have evolved different types and
patterns of sleep, dealing with different environmental needs.
The energy conservation theory states that warm blooded animals need to expend a lot of energy to
maintain a constant body temperature. As all activities use energy, sleep therefore serves the
purpose of enforced inactivity much as hibernation is a means of conserving energy. This theory has
even been denoted as the hibernation theory of sleep (Webb '82).
In contrast the foraging requirements theory thinks that time spent sleeping may be constrained by
food requirements as animals have to gather food. Herbivores spend their time eating plants that
are relatively poor in nutrients. They therefore have to spend a large period of time eating and
consequently cannot `afford' to spend time sleeping.
Another theory is predator avoidance. This theory suggests sleep is constrained by predator risks.
Predators can sleep longer whereas prey species must be more vigilant to avoid predators. As a
result sleep time is vastly reduced. Logically, they shouldn't sleep as all but sleep is vital so they must
sleep when least vulnerable.
A final explanation is the waste of time theory. This theory states that sleep helps animals to stay out
of the way of predators when they are most vulnerable in order to stay still. Usually this involves
sleeping in the darkness.
In conclusion sleep is one of the last complex behaviours of which the exact purpose remains unclear.
As a result there is no one single explanation for sleep and scientists haven't agreed over the