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The nature and function of sleep

Updated on March 29, 2013

~Please don't try to pass my work off as your own~

Sleep is not a total loss of consciousness but a descent into reduced consciousness passing through different levels of awareness and different kinds of brain activity. The sleep wake cycle is a circadian rhythm, however sleep itself is an ultradian rhythm with a number of stages, each of which lasts about 90 minutes, repeated five or six times a night. In order to enter sleep however, we have to recognise hormone, melatonin, which is produced by the pineal gland and sends a signal to regulate the sleep-wake cycle in the sleep centre of the brain. Sleep is triggered by the presence of darkness from the retina of the eye, and this is perceived in the SCN in the brain which sends a message to the pineal gland which then produces melatonin; this reduces brain activity. The levels of melatonin increase in the middle of the night and gradually fall as the night turns to morning, so exposure to light before bed can push your biological clock in the wrong direction, making melatonin ineffective.

Sleep patterns have been studied using EEGs (recordings of the electrical activity of the brain). During wakefulness, the EEG shows desynchronised brain activity, but as sleep progresses, brain patterns become more synchronised.

Once your sleep cycle begins, you enter stage one, which is where you become relaxed and brain waves are synchronised. There are characteristic alpha waves in the EEG, slow eye rolling, and reductions in heart rate, muscle tension, and temperature. This stage can be regarded as a state of drowsiness. The transition from awake beta waves to the alpha waves of stage 1 is often accompanied by a hypnogogic state, in which we may experience hallucinatory images. Stage two presents some altered patterns known as sleep spindles and K-complexes which help phase out the external stimuli and keep the sleeper asleep. The EEG waves become slower and larger but with short bursts of high frequency sleep spindles. Stages 3 and 4 are those of deep sleep: the EEG shows theta and delta waves. Dreams can occur in these stages but are less organised and harder to remember. In these stages, people are much harder to wake. Stage 5, also known as REM (rapid eye movement) sleep, presents a pattern that is significantly different from all other stages, that together are also known as nREM (non rapid eye movement) stages. In this stage, the EEG shows beta waves that are very similar to alpha waves. The brain activity becomes desynchronised, heart beat increases, and the sleeper also experiences muscle paralysis. This stage is believed to be the stage in which people dream the most and have the most vivid dreams.

After the sleeper has worked through the first four stages of progressively deeper sleep, he or she reverses the process. Stage 4 sleep is followed by stage 3, and then by stage 2. However, stage 2 is followed by REM sleep. After that, the sleeper starts another cycle, working his or her way through stages 2, 3, and 4, followed by stage 3, then 2, and then REM once again. A complete sleep cycle lasts about 90 minutes, and most of us will complete about five cycles per night, with progressively less SWS and more REM activity as morning approaches.

Sleep is unlikely to exist unless it has a function i.e. it is adaptive. Meddis (1975) proposed the evolutionary/ecological theory of sleep, arguing that it could be a time of increased safety as animals are inmobile and so less likely to be noticed by predators. Meddis proposed that the sleep behaviour shown by any species depends on the need to adapt to environmental threats and dangers. In the case of those species that depend on vision, it is adaptive for them to sleep during the hours of darkness. Those in danger from predators should sleep more of the time than those that are the predators. Allison et al (1976) found the opposite, however; they found that predators tend to sleep more than those preyed upon. This might seem inconsistent with adaptive theories of sleep, however species that are in danger from predators might benefit from remaining vigilant most of the time and sleeping relatively little. Evidence has also been found that the pattern of sleep is often dictated by the environmental threats faced by animals. Pilleri (1979) found that dolphins living in the River Indus are in constant danger from debris floating down the river. As a consequence, these dolphins sleep for only a few seconds at a time to protect themselves from the debris.

Webb (1982) suggested a different adaptive account, called the hibernation theory. He proposes that sleep is adaptive because it is a means of conserving energy in the same way that hibernation enhances survival by reducing physiological demands at a time of year when they would be hard to fulfil. The same principles could be applied to staying awake at night. Any animal that is not nocturnal may as well relax and save energy at night, and nocturnal animals will do the same during the day. Animals that don’t rest when not engaged in finding food use more energy therefore need more food, which may decrease their survival potential.

The function of sleep has also been discussed via restoration theories. Oswald (1980) identified slow wave sleep as being important in the recovery process, especially as it has been linked with the release of growth hormone and protein synthesis. There is also evidence of essential physiological processing during REM sleep. Stern et al (1974) argue that the normal function of REM sleep is to restore levels of neurotransmitters after a day’s activities. This is supported by evidence that some people on antidepressants show decreased REM, possibly because the drugs are increasing their neurotransmitter levels. The negative effects of REM rebound also support the view that REM sleep provides a restorative function. However, it is possible that restoration occurs during waking hours but is less efficient or more resource intensive.

A further explanation for the function of sleep is the psychological function. Naitoh (1975) discussed various studies concerned with the effects of 1 night’s sleep deprivation on mood. The effects were consistently negative. Sleep-deprived individuals described themselves as less friendly, relaxed, good-natured, and cheerful than those who had not been sleep-deprived. REM sleep is probably the most important stage in terms of psychological wellbeing. Dement (1960) carried out a systematic study of REM and nREM sleep. Some of his Ps were deprived of REM sleep over a period of several days, and others deprived of nREM. In general, the effects of REM sleep deprivation were more severe, including increased aggression and poor concentration. Jouvet (1967) used the flower-pot technique to test the consequences of REM deprivation in cats. He placed them on an upturned flower-pot in a large tank of water. The cats eventually fell asleep; in nREM sleep they were able to remain upright, but with the loss of muscle tone that comes with REM, they slipped into the water and were awoken. They began to wake up as soon as their heads began to nod. In the end, the cats died.

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