Outline one or more evolutionary explanations for the functions of sleep

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There are 4 main evolutionary theories explaining sleep as an adaptive response to aid survival.

The first explanation is for energy conservation. Warm blooded animals must expend a lot of energy maintaining a constant body temperature. This can be problematic for smaller animals with higher metabolic rates as they use even more energy. Evolutionists would argue that sleep therefore serves the purpose of enforcing a period of inactivity to conserve energy, as metabolism lowers when asleep. 

Zeppelin shows support as they found that smaller animals with higher metabolic rates did sleep more than larger animals, supporting the idea of energy conservation for sleep. However, there are many exceptions to this such as sloths that sleep for 20 hours a day, criticising conservation theories.

Sleep may be used by some animals for energy conservation by this may be different from one species to another due to adaptive pressures. But this is not the case with humans as no such predator risks are evident, yet we spend a third of our lives sleeping.

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It is important to realise the distinction between REM and NREM for energy conservation. The energy consumption of the brain drops only in NREM sleep; during REM the brain is still relatively active. This leads to suggest that it is only NREM sleep that has evolved for energy conservation. It is also argued that animals that are more primitive only have NREM sleep. REM sleep appears to have only evolved about 50 million years ago in mammals. It may be that NREM sleep evolved first for energy conservation, whereas REM sleep may have evolved later to maintain brain activity. This ...

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