Evolutionary theories of sleep
The theories – AO1
According to evolutionary theories, sleep itself isn’t _____________ instead it is important
for what it prevents us from doing.
Hibernation theory (Webb, 1974) states that sleep gives a survival advantage because it helps
conserve energy by:
 Reducing energy spent on behavioural activity (things we cannot do) because we are
___________________.
 Reducing energy spent as _______________ since sleep causes a drop in body
temperature and metabolic rate
Hibernation theory predicts that sleep will _________________ when the potential for energy
loss whilst awake is great – for instance when it is cold. It also predicts that sleep will increase when
energy intake goes down – for instance when food is scarce – because energy needs conserving even
more.
Predation theory (Meddis,1975) explained the amount of sleep an animal has by considering:
 its exposure to predators, including whether it has a ____________ place to sleep
 its need for food and how much of the day it needs to eat for.
Animals that are at risk from predators and cannot find a safe place to sleep, have high metabolic
rates that require much food gathering and sleep very __________. Short tailed shrews have a safe
burrow but also have a high metabolic rate so must eat around the clock or die. They get
________________ sleep per day. Prey animals such as sheep, who do not have a safe burrow
and who have to eat for much of their time sleep for about _________ a day. Lions sleep for much
of the day because they do not have predators and they also need to eat _______________.
The theory also explains when animals sleep. Humans have poor night ___________ so it makes
sense that they sleep at night, when they would be at greater risk; sleep essentially keeps them out
of trouble. Cats however can see quite well at night so it is an _________________ for them to
hunt at night and sleep in the day. Animals keep __________ whilst asleep to attract little attention
from predators.
Advantage
4 hours
asleep infrequently
2 hours
vision
vital
safe
increase
still
little
heat
IDA- Evolutionary approach
There is no doubt that sleep may have an evolutionary survival advantage and this theory is logical to
some extent but there are also issues with using this approach to explain sleep. This is because it is
virtually impossible to test experimentally and we cannot conclude whether these processes actually
resulted in the sleep patterns we observe today. The theory lacks scientific validity and amounts to
just speculation.
This is illustrated by the differences in the theories of Webb and Meddis; one suggests that we sleep
to conserve energy, while the other suggests we sleep to stay safe from predators. This makes it
difficult to identify the precise evolutionary advantage behind the behaviour.
Other approaches could also explain why we sleep, such as………..
Can sleep be better explained using
an alternative approach to the
evolutionary one? E.g. biological,
AO2- Evaluation of the theories
Research evidence
Alison and Cicchetti (1976) analysed the sleep patterns of 39 animal species and found a
relationship between risk of predation and amount of sleep. Animals with a high risk of predation
slept less than those with a lower risk.
Ground this evidence to the theory, remembering to state which evolutionary theory it links to
In winter when food is scarce and temperatures drop, some species hibernate.
Ground this evidence to the theory, remembering to state which evolutionary theory it links to
Challenging evidence- Think of some evidence about sleep that you know that challenges the theory –
explain why it is a challenge for the theory.
Every species of animal still sleeps – even shrews that may be better off not wasting time on sleep - and
some have evolved peculiar sleep patterns (dolphins sleep one hemisphere at a time). Why are these facts
also problematic for the theory.
AO3
Wider evaluation
How good are the evolutionary theories
of sleep? You may also want to consider
whether the theories themselves have the
qualities of good science.
Why might this theory not be so
applicable to humans anymore?