"Sleep. Those little slices of death. How I loathe them." Edgar Allan Poe (1809-49).
Is sleep an absence of life, like Edgar suggested, or is it a complex behaviour with specific functions?
The latter seems more likely as humans spend about one third of their lives asleep.
These theories take the view that sleep is an adaptation that has evolved to fit in with the animal's ecology.
Meddis (1979) suggested that sleep patterns depended upon the animals foraging and predator avoidance behaviours:
|Type of animal:||Sleep pattern:|
|Grazing animals - often foraging, also at risk from predators.||Little sleep - vulnerable and need to eat.|
|Hunting animals - infrequent foraging, at little risk from predators.
||Lots of sleep - safe and can save energy.|
Webb (1982) supported the view that sleep helps to conserve energy but also thought that it allowed animals to become inconspicuous and avoid predators.
Aquatic mammals are in danger of drowning if they fall asleep so may sleep for a few seconds at a time (for example, Indus dolphin) or allow only half of the brain to sleep at any one time (for example, Bottlenose dolphin).
However, there are problems associated with these explanations for sleep:
- They are only really explanations of how animals fit sleep in to their lives.
- Remaining still could also save energy and help to stay inconspicuous.
- Do not explain function of complex neural activity during sleep.
These theories state that sleep is necessary to restore biological processes in the body.
The fact that a growth hormone is released during Stage 4 sleep supports this claim.
Oswald (1980) suggested that the activity involved in REM sleep (paradoxical sleep) is required for brain restoration and Stage 4 sleep for body restoration. Horne's (1988) view partially agrees with Oswald's but he saw both REM and Stage 4 sleep as significant in brain restoration. This view is supported by evidence that heavy exercise (presumably leading to body exhaustion) does not lead to extra time spent asleep.
Sleep deprivation studies have been performed to work out what sleep does for us, although the results are inconclusive.
Problems such as impaired attention, confusion and delusions are associated with a loss of sleep (Hüber-Weidman, 1976), but these are not permanent.
Research into physical harm due to a lack of sleep is limited to animal and case studies (due to ethical constraints of sleep deprivation in humans), which provide fairly weak evidence.
Much sleep deprivation research has focused on the restorative nature of REM sleep.
Unlike total sleep deprivation, REM sleep deprivation (or REM starvation) causes the sleeper to increase the frequency of REM sleep each night. This REM rebound effect suggests that REM sleep has an important function as sleepers try to catch up on what they have missed (Dement, 1960).
If REM sleep is necessary, what does it do?
Oswald's (1980) theory of brain restoration and growth is supported by the fact that REM sleep drops with age (infant's brains need more REM than adults) and increases with injury.
There are alternative theories to restoration. It is possible that the high level of brain activity during REM sleep serves to consolidate information learned during the day (Empson & Clarke, 1970).
OK, here's a short task for you to review the main theories of sleep. Drag the terms and drop them in the correct cells: