Evolutionary Theory of Sleep

Notes I made for my A2 exam.

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Evolutionary Theory
of Sleep
Sleep must be adaptive as every an animal does it despite substantial costs, it must also be
advantageous and a vital bodily function.
Predator avoidance- sleep constrained by risk of predators. If an animal is a predator it can sleep for
longer as there is no fear of being eaten. However if an animal is prey its sleep time is reduced so it
can watch out for predators.
Energy conservation- (Webb) Hibernation theory: sleep provides a period of enforced inactivity
where animal can save up energy, as mammals use a lot of energy maintaining vital bodily functions
such as temperature. Animals with a high metabolic rate also use up a lot of energy and therefore
sleep for longer. ....sloth... REFUTING- The sloth which is very large and slow, meaning it has a low
metabolic rate is shown to sleep for 20 hours a day, this may be due to the fact that it is wasting time
in order to prevent injuring itself.
SUPPORTING (energy conservation)- Zepellin and Rechstaffen 1974- relationship between sleep
length and body size (the larger the animal the lower the metabolic rate) in 53 mammalian species.
Body size negatively correlated with amount of sleep. This suggests that animals with higher
metabolic rate sleep for longer. METHODOLOGICAL- Sample size quite small in relative comparison to
the vast number of animals there are.
Foraging requirements- sleep constrained by food requirements. Herbivores such as cows and
horses eats foods such as grass that are low in nutrients and therefore a lot of it needs to be eaten
to maintain energy meaning that they cannot afford to spend long periods of time asleep. Carnivores
on the other hand eat foods rich in nutrients and are only required to eat occasionally, therefore can
spend more time asleep conserving energy.
SUPPORTING (predator avoidance)- Allson and Cicchetti 1979 studied animals in a lab looking at the
relationship between amount of sleep and the typical degree of danger experienced by that
species. They found amount of danger negatively correlated with the amount of sleep. This supports
the predator avoidance idea that the more risk you are of predators the less sleep is taken.
METHODOLOGICAL- The data might not be fully valid as the animals were tested in lab conditions and
therefore might not display their natural behaviour.
SUPPORTING- It has also been found that predators sleep more prey again supporting predator

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Waste of time- Meddis 1975 proposed the "waste time hypothesis" claiming that sleep helps
animals stay out of harm when they are most vulnerable. It also ensures that animals stay still when
they have nothing better to do and also find places to sleep where they are hidden therefore
reducing the risks of being eaten .
Siegal claimed that being awake is riskier than being asleep because an animal is more likely to be
injured.…read more


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