The evidence from sleep deprivation studies suggests some negative effects.
Oswald identified SWS as being important in the recovery process, especially because it has been linked with the release of a growth hormone and protein synthesis.
Stern and Morganne (1974) propose that the function of REM sleep is to restore levels of neurotransmitters after the day’s activities. This evidence is supported. People on antidepressants show decreased REM; this is probably due to the drugs increasing their neurotransmitter levels. Therefore, this evidence suggests that REM sleep provides a function of restoration.
However, restoration occurs during waking hours but is less effective. Loss of sleep would have an effect but not necessarily a serious one. Sleep deprivation effects alone are not sufficient evidence.
Evidence of the effects of physical exertion are supported by Shapiro et al. They studied runners who had taken part in an ultra marathon. They slept for about an hour and a half longer than normal on the two nights after the run, with a larger increase in the SWS4 sleep. This evidence tells us that SWS4 sleep would be for restoration.
Horne and Minard tried to exhaust their participants, and found they went to sleep faster but not for longer. It is purposed people who take little exercise would sleep less than those who take an average amount of exercise. There is little evidence for this.
There is a considerable amount of support for this study but also problems or inconsistencies. If restoration was the only function of sleep we would expect to find consistent effects from sleep deprivation. The inconsistent effects may be due to the fact that only some aspects of sleep provide a physiological function. Also if this was the case, we would expect people who are more active to require more sleep, so therefore a mix of findings cannot full support this.
The theory however, does support that it is impossible to go without sleep and remain ok, this shows face validity. The importance of SWS4 sleep and REM is also supported. After sleep deprivation there was a greater recovery of REM sleep and SWS4 sleep. This can bring to conclusion that the function of sleep is vital for restoration of the body and brain, but due to lack of evidence, whether this is the main function is questionable. The recovery approach seems to provide a more thorough and well developed account of sleep compared to other theories. It could be argued that the recovery approach provides views on why sleep is important.
According to theorists, sleep can be regarded as an adaptive behaviour favoured by evolution. Evolution is a fact. The key principle is survival. Animals that survive must have adaptive characteristics that have enabled them to survive. It is likely that sleep is some way adaptive. Evolutionary theorists presume that sleep occurs in all animals because it promotes survival and reproduction. Each animal adapts their sleep to suit their environmental demands.
Meddis proposed that the sleep behaviour shown by any species demands on the need to adapt to environmental threats and dangers. Therefore, sleep serves the function of keeping animal fairy immobile and safe from predators during periods when they cannot engage in feeding. For animals that depend on vision, it is adaptive for them to sleep during the hours of darkness.
Those species in danger from predators sleep more of the time than those species that are predators. In fact, predators actually sleep longer, as shown by Alison at al. this seems inconsistent with adaptive theories of sleep. However, those at more danger from predators may benefit from keeping themselves vigilant. It is impossible to falsify Meddis’ theory.
To evaluate, we have already seen that there are problems with the predation theory as it is non-falsifiable. You cannot possibly predict an evolution change. There are also problems with applying this theory to human sleep. If the function of sleep was due to evolutionary factors why hasn’t there been some move in the direction of less sleep when today there would be enormous advantages for an individual who needed very little sleep. The explanations in this theory are only of how animals fit sleep in to their lives. The theory also does not explain the function of complex neural activity during sleep.
In conclusion sleep is generally less crucial according to adaptive theorists, in contrast to the recovery theorists who believe sleep is absolutely essential. Horne made the point that sleep probably serves different purposes in different species. Therefore no theory is likely to be adequate. It could be argued that the adaptive approach focuses on when different species sleep.
