Functions of Sleep: Evolutionary (Ecological) Explanations

The hibernation theory of sleep.

Warm-blooded animals (mammals), such as ourselves need to expend a lot of energy to maintain a constant body temperature.

Sleep, however, serves the purpose of providing a period of enforced inactivity (therefore using less energy) much as hibernation is a means of conserving energy.

This is particularly problematic for small animals with high metabolic rates, such as mice (metabolism refers to the chemicals processes taking place in the body). All activities use energy, and animals with high metabloic rates use even more energy.

If sleep is a necessity, the time spent sleeping may be constrained by food requirements. An animal has to gather food.

Herbivores spend their time eating plants (such as grass) that is relatively poor in nutrients. As a result, they must spend a great deal of time eating, and consequently cannot 'afford' to spend time sleeping.

Carnivores, such as cats and dogs, eat food that is high in nutrients, and so do not need to eat continuously. Therefore, they can 'afford' to rest much of the time, and by resting they can conserve energy.

A further likelihood is that sleep is contrained by predation risk. If an animal is a predator, then it can sleep for longer, whereas for prey species, their sleep time is reduced as they must remain vigilant to avoid predators. Logically, to be safe they shouldn't sleep at all but if sleep is a vital function then they are best to sleep when least vulnerable.

The 'waste of time' hypothesis.

He suggested that sleep helps animals to stay out of the way of predators during the parts of the day when they are most vulnerable. For most animals, this means sleeping furing the hours of darkness. It also means sleeping in places where they will be hidden. According to Meddis, sleep may simply ensure that animals stay still when they have nothing better to do with their time.

Siegel concurs with Meddis' view: he points out that, in fact, being awake is riskier than sleeping because an animal is more likely to be injured. Siegel's view is that the only possible explanation for sleep is that it enable both energy conservation and keeping an individual out of danger: 'in the wild, the best strategy for passing on your genes is to be asleep for as lng as you can get away with.. and that is exactly what you see'.

The little brown bat is awake for a few hours each day, just when the insects that it lives on are awake. It might be expected that the bat would sleep little because it is a small mammal - but it doesn't. It is awake when it needs to be.

The data for such sleep patterns are not always reliable:

- Some records from studies are conflicting, eg. the various studies of giraffes' NREM sleep is somewhere between 2.5 and 3.6 hours.

- Often data is based on fewer than five animals per study and their sleep has been stufied under lab conditions where they may not be displaying natural behaviours.

- The animals may also have been observed for 12 hours.

To compare sleep habits across different species.

Zepelin and Rechtschaffen found that smaller animals, with higher metabolic rates, sleep more than larger animals. This supports the view that energy conservation might be the main reason for sleep.

There are many exceptions, such as sloths, which are very large yet sleep 20 hours a day.