Evolutionary theories of sleep

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Animals and sleep- they enter periods of quiescence


Quiescence might be described as sleep- and the actual behaviour varies across species but all animals engage in something identified as sleep –which is supported by Cirelli and Tononi (2008). Elephants do this standing up and cats with their eyes half open- but in doing so both animals are sleeping.


Some animals sleep for prolonged uninterrupted period and others in shorter bursts. The content with animals differs in the amount of NREM and REM and others only have REM.


Some animals don’t sleep in the conventional sense e.g. dolphins- because they have to keep moving. So the have evolved a way to allow one half of their brain to sleep at a time! This is called unihemispheric sleep.


This may suggest that there is a strong evolutionary pressures shaping sleep patterns placed on animals. Since each animal faces it own evolutionary pressures- the variation across sleep- observed across species- may reflect different adaptive responses to these pressures!


Theories focus on:

-benefits of reduced energy expenditure during sleep,

-responses to predation,

-and adaptive functions of certain types of sleep.


All theories have in common the fact that- sleep has evolved because it serves a purpose useful to fitness of the species.



(Hibernation theory of sleep)

Main ideas:

Webb (1982)

      Function of sleep similar to hibernation

      A mechanism evolved to force animals to save energy at times when it would be relatively inefficient to be awake

      Animals which are adapted to diurnal (daytime) activity and eat during the daytime would be wasting valuable energy at night when they would be awake and unable to eat

      Sleep therefore reduces the amount of energy wasted by animals


Berger (1995)

-      Sleep helps to offset the greater cost of being endothermic (warm-blooded).

-      Extended bouts of sleep reduce metabolic pressures on animal

-      An endothermic (warm-blooded) animal at rest has a metabolic rate up to 10 times greater than an ectotherm (cold-blooded animal) in a similar state.


Key words: Ectotherm (cold-blooded animal). Endothermic (warm-blooded).



Key points:

+ve: Theory supported by hibernation and


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