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Blue: AO2 - Evaluation - studies
Red: AO2 - Evaluation - evaluative points/IDAs
Evolutionary theories of sleep
Meddis proposed the predator-prey status theory, claiming that sleep evolved to keep prey hidden and safe from predators when normal adaptive activities such as foraging are impossible - such as at night for diurnal animals, and in the day for nocturnal animals. Therefore, the hours of sleep required are related to an animal's need for and method of obtaining food, as well as their exposure to predators. Factors other than predator-prey status that can affect sleep behavior include sleeping environment and foraging requirements. Sleep evolved to ensure animals stay still and out of the way of predators when productive activities are impossible, so the higher the vulnerability to predation, the safer the sleep site, and the lesser the time required to spend foraging, the more time an animal should spend sleeping.
This explanation is supported by the fact that animals are often inconspicuous when sleeping - taking the time beforehand to find themselves adequate shelter to keep them hidden from predators. This also explains the early stages of the sleep cycle, "light sleep", as a transitional phase from wake to sleep, allowing the animal to ensure their own safety in their immediate environment before completely losing their alertness.
A study by DeCoursey also supports this explanation. 30 chipmunks had their suprachiasmatic nuclei (a part of the brain involved in regulation of the sleep/wake cycle) removed, and were released into the wild. All 30 chipmunks were killed by predators within 80 days, suggesting that sleep patterns are vital in ensuring the safety of an animal in its natural habitat.
A strength of DeCoursey's study was the scientific validity provided by the use of control groups, treating psychology with rigorous scientific methodology. Three groups of chipmunks were used: one with SCN damage, one who had brain surgery but no SCN damage (to control for the stress of brain surgery) and a healthy control group. The use of these controls mean that cause and effect can easily be determined - it can be reliably established that circadian disruption due to SCN damage increase the risk of death due to predation.
However, a study by Allison and Cicchetti challenges this explanation, finding that on average, prey sleep for fewer hours a night than predators - Meddis suggested the opposite trend, so his theory conflicts with these results.
The predator-prey status theory of sleep is holistic, compared to Webb's hibernation theory. Rather than only focusing on one factor, (status), Meddis suggested that several factors other than this can influence sleep behaviour, such as site of sleep (whether it's enclosed in a nest or a cave, or exposed on prairies or plains) and foraging requirements (whether it requires lots of grazing on nutrient-poor found sources, or relatively few hours gathering nutrient-rich foods such as nuts or insects.) A holistic theory that takes into account multiple factors is likely to be able to provide the best explanation for the complex behaviour that is sleep.
A problem with explaining sleep as a means to safety from predation is that many species may actually be far more vulnerable during sleep, and it would be safer to remain quiet and still yet alert. However, some species have adapted to this need for vigilance: porpoises only sleep one brain hemisphere at a time, while mallards sleep with one eye open to be able to see potential threats. The phenomenon of snoring also challenges this explanation, as it is likely to draw attention to the otherwise inconspicuous sleeping animal, and increase their risk of predation.
Webb proposed the hibernation theory, claiming that sleep evolved as a way of conserving energy when hunting or foraging were impossible. This theory suggests that animals should sleep for longer if they have a higher metabolic rate, as they burn up energy more quickly, so are in greater need of energy conservation. Conservation of energy is best carried out by limiting the brain's sensory inputs, i.e. sleep.
Berger and Philips found that sleep deprivation causes increased energy expenditure, especially under bed rest conditions. This suggests that sleep does conserve energy, and is especially useful when you're not doing normal activities.
Studies have found a positive correlation between metabolic rate and required sleep duration - small animals such as mice generally sleep for longer than larger animals, supporting the idea that sleep is adaptive as a form of energy conservation.
In times of hardship, such as when food is scarce or the weather too cold, animals sleep for longer, suggesting that sleep helps them conserve all the energy they can when resources are scarce and every calorie is critical for survival.
However, not all organisms follow this general trend, and there are some extreme outliers that challenge this theory. The sloth, a relatively large animal with a slow metabolic rate sleeps for approximately 20 hours a day, challenging the general trend that Webb's theory.
REM sleep, characterised by high levels of brain activity, actually uses the same amount of energy as waking. If REM sleep did not serve some other purpose, it would be maladaptive, as it does not help conserve energy due to the high levels of brain activity.
Overall evaluation of evolutionary theories of sleep
Generally, evolutionary theories of sleep are holistic, looking at the entire lifestyle of an animal rather than single factors in an attempt to explain and predict sleeping behaviour. Holistic, complex theories are likely to be able to provide the best full explanation for the complex behaviour that is sleep.
Much of the evidence in support of this approach is based on observation of captive animals, rather than animals in the wild. It may not accurate reflect natural animal behaviour, so these studies may lack validity. Also, these theories are impossible to test through experiment or observation, as evolution happens over thousands of years, so it is not particularly scientific.
Finally, this approach is overly deterministic, seeing sleep behaviour in humans as well as animals as being entirely caused by our evolutionary past, with no role for free will. This is an oversimplification - there is evidence to suggest that free will can play a role in influencing biological processes such as sleep.
Restoration theories of sleep
Oswald's theory is supported by the findings that newborn babies spend large amounts of time in proto-REM sleep (a third of every day.) This is a time of massive brain growth, with the development of new synaptic connections requiring neuronal growth and neurotransmitter production. REM is a very active phase of sleep, with brain energy consumption similar to waking, so Oswald's theory can explain this phase and why it's so dominant in newborns.
Oswald also found that sufferers of severe brain trauma such as drug overdoses spend much more time in REM sleep. It was also known that new skin cells regenerate faster during sleep - Oswald used these results to conclude that REM sleep is for restoration of the brain, and NREM sleep is for restoration of the body.
Jouvet (1967) placed cats on upturned flowerpots surrounded by water, which they would fall into upon entering REM sleep. Over time, the cats became conditioned to wake up upon entering REM sleep, depriving them of the vital fifth stage of sleep. The cats became mentally disturbed very quickly, and died after an average of 35 days. This supports Oswald's theory: the cats had NREM sleep and suffered no obvious physical ailments, buts died from organ failure brought on by brain fatigue, resulting from the lack of REM sleep.
Jouvet's use of non-human animals raises an important issue. As well as being potentially considered unethical due to the extreme cruelty inflicted upon the animals for relatively little in the way of socially important results, the use of cats is a problem due to physiological differences in the mechanisms controlling sleep in humans and cats, meaning that it is anthropomorphic to generalise the results to humans.
Horne's restoration theory suggests that REM and deep NREM sleep are essential for normal brain function, as the brain restores itself in these stages of "core sleep." Light NREM has no obvious function - Horne refers to it as optional sleep, that might have had a role in keeping the animal inconspicuous by ensuring safety before its progression to deep sleep. Entering NREM causes a surge in growth hormone release - but this is unlikely to be used for tissue growth and repair, as nutrients required will have already have been used. He therefore theorises that bodily restoration takes place in hours of relaxed wakefulness during the day, when energy expenditure is low and nutrients are readily available.
Supporting evidence for Horne's theory comes from sleep-deprived participants given cognitive tasks to carry out. They can only maintain reasonable performance through significantly increased effort, suggesting that sleep deprivation causes cognitive impairment because the brain has not had enough sleep necessary to maintain prime cognitive function.
Radio DJ Peter Tripp managed to stay awake for 8 days (200 hours). During this time he suffered delusions and hallucinations so severe it was impossible to test his psychological functioning. It is thought that sleep deprivation caused these effects as the brain was unable to restore itself. This supports Horne's theory, as having no REM or NREM lead to cognitive disturbances, rather than any physical impairment.
Randy Gardner remained awake for 11 days (264) hours, suffering from slurred speech, blurred vision and paranoia. He had fewer symptoms than Tripp despite being awake for longer, and soon managed to adjust back to his usual sleep pattern after the experiment. This again supports Horne's theory - slurred speech, paranoia and blurred vision are likely to be a result of neurological rather than physical impairment due to lack of core sleep.
Both Tripp and Gardner's studies are case studies, meaning they lack generalisability to a wider population. The massive individual differences found between only two case studies suggest that individual differences alone play a large role in how the individual experiences sleep, and how much sleep they need, so individual differences affect sleep too much to draw any valid conclusions from case studies.
Also, Tripp and Gardner were both male. research has shown that hormonal differences and levels can play a large role in determining how the individual experiences sleep, so, taking into account hormonal differences between genders, it would be beta bias to attempt to generalise their results to females specifically.
Finally, a methodological issue in Gardner's study comes from the observation of symptoms like blurred vision. It is difficult to establish whether this has a psychological or physiological cause, as it could either a result of bodily impairment such as a malfunction of the optic nerve, or brain impairment such as occipital lobe malfunction, the part of the brain responsible for visual processing. This makes it difficult to establish what damage was done by the sleep deprivation - physical and mental as Oswald would suggest, or purely mental, as Horne would suggest?
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