Restorative theories known as homeostatic theories are the most intuitive ones as they suggest that the function of sleep is rest and recuperation in order to restore the body to full working capacity (Horne) During SWS, particularly stage 3 & 4 growth hormone is released into the blood in large quantities which helps to restore bodily tissues, suggesting that sleep serves a restorative function but there are two debates as to the precise nature this might take.
Oswald’s theory suggests the high levels of brain activity during REM reflect brain recovery while an increase in the body’s hormone activity during SWS reflects restoration and recovery in the body. Oswald views REM sleep as essential for the brain repair and restoration, this is supported by the high levels of REM sleep seen in new born babies where it makes up 50-60% of sleep time gradually falling to about 25% as the child grows.
Horne’s model is similar to Oswald but he concluded that sleep deprivation in normal participants produces only mild effects that were not dramatic it did however involve some problems with cognitive abilities such as perception, attention and memory. Horne therefore proposes that ‘core sleep’ consisting of stage 4 SWS and REM sleep is essential for the normal brain functioning for our cognitive abilities while the lighter stages of SWS are not essential. During the core sleep the brain recovers and restores itself after the activities of the day.
Stern & Morgane believe during REM sleep neurotransmitter levels are replenished supporting the idea of restorative sleep backed up by anti depressants increasing neurotransmitter levels, reducing REM activity. The main difference between Oswald and Horne concerns the proposed functions of slow wave sleep as total sleep deprivation produces few obvious effects Horne believed that body restoration is not the purpose of sleep. He suggests it occurs during periods of relaxed wakefulness, leaving core sleep to provide for the restoration of brain systems.
Horne applies his study to human participants and it is possible the theory put forward but Oswald could apply to non-animals as there are significant differences between species and it is possible that no single hypothesis could cover them all. Ultimately for most people intense exercise may cause them to fall asleep more quickly but it does not cause them to fall asleep for longer which is demonstrated by the giant sloth that sleeps for 20 hours a day and has very little energy expenditure.
(Breedlove. ) Also sleep deprivation studies have failed to provide conclusive evidence that sleep is necessary to keep the body functioning, Horne reviewed 50 studies and very few reported deprivation had an interference with the participants ability to perform physical exercise and there was no evidence of a physiological stress response to the deprivation.
There is a flaw to the restoration theory as Horne pointed out, that amino acids are only freely available five hours after meals and most people eat several hours after going to bed, therefore implying not much protein synthesis would go on during sleep. Furthermore the support for this theory comes from animal studies that found prolonged sleep deprivation of rats led them to die within 19days due to the interference with the immune system but to keep them awake they must be constantly stimulated and therefore stressed.
This research suggests that sleep provides the only opportunity for restoration in some species. Moreover fatal familial insomnia is a rare inherited defect, preventing people sleeping when they become middle aged leading to death within two years. Autopsies revealed degeneration of the thalamus suggesting patients have brain damage and making it hard to generalise the findings. Ultimately research remains inconclusive and the findings are inconclusive therefore lacking reliability and thus the validity of the research must be questioned.
Evolutionary theories of sleep suggest it is evolved to aid survival in the evolutionary past, i. e. to conserve energy at times it would be inefficient to be awake and to keeps animals out of harm’s way from predators (Webb) However mammalian physiology does not vary greatly but yet the time animals of different species spend asleep differs greatly for example foxes for 9 hours, sloths for 20 hours and cats for 14. 5 hours and surely if restoration was the main function of sleep then the various species should sleep for a similar amount of time.
Furthermore there it is not clear why such a complex mechanism as sleep would evolve simply to keep vulnerable animals out of harm’s way when behavioural inactivity would do the same thing i. e. freezing. Furthermore sleep is found in species that would seem to be better off without it (Carlson) for example, dolphins in Pakistan have become blind because night vision is unnecessary and so if sleep served an adaptive function then surely it would have been elimated through the process of natural selection.
Ultimately this theory is deterministic as it implies we are slaves to inborn instincts and unable to use free will, it is also reductionist as it fails to offer a complete explanation of the nature of sleep. People generally are told to ‘sleep on it’ when making big decisions suggesting we are better decisions after a period of sleep which is what the brain plasticity theory deems to be the case and according to this theory sleep aids processes such as memory consolidation as learning and long term memory formation are related to changes in neurons that occur during sleep.
The simple idea is that new experiences are put into LTM during sleep as the animal is not controlling any behaviour. We can make two direct predictions: some forms of LTM should improve following sleep and LTM should be impaired in the sleep deprived. The main research into brain plasticity is by Hahn, Mehta & Sakmann who found that information is first stored as ST memories in the hippocampus and is transferred to the cerebral cortex where they enter long term memory during deep sleep.
Therefore if a person doesn’t get enough sleep they become overloaded with new incoming information making them confused, therefore giving this theory face validity. Also the little evidence there is to support this theory is scientific based providing objective quantitative data and is less likely to be subject to researcher bias, therefore giving scientific validity. However the theory is incomplete as it does not explain what happens to the disgarded information and is therefore reductionist.