All this is evidence that sleep serves important physiological functions. Oswald (1980) proposed that sleep is the time in which most tissue growth and repair occurs and that it is therefore necessary. Others have argued that this growth and repair can take place during periods of micro-sleep, however, and that extended periods of sleep are not necessary. It is worth noting, though, that babies spend a much larger proportion of the day in sleep compared to adults and so perhaps this allows the rapid growth (especially of brain tissue) which occurs in the first months of life to take place.
Stern and Morgane put forward an alternative restoration theory, proposing that during sleep, the levels of neurotransmitters in the brain are restored. Again, this theory would seem plausible, in part because people who take antidepressants (these drugs help restore neurotransmitter levels) appear to need less REM sleep than normal.
Evolutionary theories would perhaps seem less likely than restoration ones in terms of explaining the apparent need for sleep. According to hibernation theory (Webb, 1982), sleep has evolved to enable energy to be saved at times when being awake and active would be of little use to animals (e.g. because it is too dark for them to do anything). However, this does not seem to be applicable to humans – in order to meet the current demands of society (such as work and social activity), being awake all of the time would be of extreme value, especially as electric lights and so on mean that 24 hour activity would be possible. It is difficult to see, then, how sleep could be advantageous as Webb suggested. Neither can Meddis’ (1975) theory be easily applied to humans. He suggested that sleep evolved so that animals can remain inactive and, consequently, hidden at times when they are more likely to be predated. As humans are the species highest in the food chain, it is unlikely that this best explains why we sleep. It is questionable whether this theory can even be applied to non-human animals. It has been found that predators generally have more sleep per 24 hour period than animals of prey. This is not consistent with the theory, and would suggest that restoration theories are more plausible – predators typically expend more energy than non-predators and so they may need more sleep to replenish this energy.
It has been suggested that although restoration theories are probably better in explaining why we sleep, evolutionary theories may be useful in explain when we sleep, and for how long. This is supported by the fact that in some countries, where the sun is at its hottest at around midday (e.g. Spain) and it is too hot to safely be out in the sun, siestas are taken and people adapt well to this sleep pattern, having an extended period of sleep for only about 5 hours per night as opposed to the average 7 to 8 hours per night in other countries.
It has also been proposed that sleep is important for psychological respite and restoration. As with theories of physical restoration, this has received empirical support. It has been found, for example, that after a night of poor sleep, individuals report higher levels of anxiety and unless highly motivated, perform worse on tasks which require extended periods of concentration.
Perhaps some stages of sleep are important to the physical and psychological wellbeing of humans, but others aren’t. Stage 4 and REM sleep are the deepest stages of sleep, and the ones which have been proposed to be most associated with tissue growth and repair and with neurotransmitter replenishment. In support of this, studies of sleep deprivation have revealed that when participants are allowed to sleep normally after periods of no or reduced sleep, only about 25% of lost sleep is recovered, but around 70% of REM is caught up on. In this REM sleep, dreams are most likely to occur and so if, as has been proposed, dreams serve important functions (such as getting rid of ‘parasite information’ so that neural connections can be freed, or solving problems which may be being experienced in waking life), then it would follow that REM sleep is, in fact, essential.
It would seem that most evidence points towards sleep being a necessary phenomenon but psychologists are not able to give exact reasons why. Although studies give good insight into and provide useful information about the nature and purposes of sleep, it is unethical to deprive participants of sleep for too long and so possibilities for research are limited. Nevertheless, most theories of sleep have been supported in some way and so it is likely that sleep has evolved for a variety of reasons and serves a number of different physiological and psychological functions.
