Sleep is the fundamental anabolic process common to all life forms, plant and animal. In animals the anabolic state – sleep – is characterized by an absolute minimal degree of consciousness and decreased responsiveness to the surrounding world. Sleep occurs cyclically, roughly once every 24 hours even though the average human inner body clock usually runs a 24.5-25.5 hour cycle. The cycle is reset daily (that is, every 24 hours) by various stimuli, the level of sunlight being the most obvious example. One of the correlates of this cycle is the level of melatonin, which is high at times when we tend to sleep.
Although one sign of sleep is loss of consciousness, sleep is very different from other forms of unconsciousness such as coma. During sleep, the brain remains very active, going through a number of well defined stages, and sleep can always be reversed given enough external simulus.
In some cultures it is common to sleep twice every 24 hours (afternoon nap, siesta).
Sleep also refers to the mucus that collects around the eyes during sleep as defined above.
A Clinical Analysis – Stages of Sleep
Human sleep is usually divided into 5 stages according to electroencephalographic (EEG) recordings:
- Rapid Eye Movement (REM) sleep with rapid eye movements
- Stage 1 with 50% reduction in alpha waves compared to awake resting with eyes closed. The stage is sometimes referred to as somnolence or “drowsy sleep”. It appears at sleep onset and can be associated with so-called hypnagogic hallucinations
- Stage 2 with “spindles” (12-16Hz) and “K-complexes”
- Stage 3 with delta waves, also called delta rhythms (1-2Hz) 20%-50% of the time.
- Stage 4 with delta waves over 50% of the time
Animals vary widely in their amounts of sleep, from 2 hours a day for giraffes to 20 hours for bats. Generally, required sleeping time decreases as body size increases. Cats are one of the few animals that do not have most of their sleep consolidated into one session, preferring instead to spread their sleep fairly evenly throughout the day.
Seals and dolphins “sleep” with alternate hemispheres of their brains asleep and the other awake. Seals need to do this so they can breathe above water while sleeping. Migratory birds also seem to sleep this way.
Even fish and fruit flies appear to sleep. If fruit flies are repeatedly disturbed so that they can not sleep, later when allowed to sleep they will stay inactive for a longer period of time.
Many animals hibernate in a deep sleep during winter to save warmth and energy. A similar kind of sleep is estivation, which is hibernating to escape the heat of summer.
What is sleep?
There is much debate and little understanding of the evolutionary origins and purposes of sleep. Virtually all who study sleep today theorise that one of the major functions of sleep is consolidation and optimization of memories (including “unlearning”). However, this does not explain why sleep appears to be essential or why mental functions are so grossly impaired by sleep deprivation.
One idea is that the state of sleep is an anabolic state marked by physiological processes of growth and rejuvenation of the organism’s immune and nervous systems. Studies suggest sleep restores neurons and increases production of brain proteins and certain hormones. In this view, the state of wakefulness is a temporary hyperactive catabolic state during which the organism acquires nourishment and procreates. Answering the question “Why do we awaken” puts us in the correct perspective to understand that sleep is the essential state of life itself. Anything that any organism does while awake is superfluous to the understanding of life’s metabolic processes, the two balancing states of sleep and wakefulness.
In support of this idea, one can argue that adequate rest and a properly functioning immune system are closely related, and that sleep deprivation compromises the immune system by altering the blood levels of specialized immune cells and important proteins called cytokines, resulting in a greater than normal chance of infections.
However, this idea is not without its critics, which point out that the human body appears perfectly able to rejuvenate itself while awake, and that the changes in physiology and the immune system during sleep appear to be minor. In addition, the brain appears to be as active during sleep as it is when it is awake.
Sleep proceeds in cycles of NREM and REM phases. Each phase has a distinct physiological function. Dreaming, for example, appears to occur during REM sleep.
Adenosine, a nucleoside which plays various roles in biochemical processes, gradually accumulates in the human brain during wakefulness but decreases during sleep.
Some drugs, such as alcohol and sleeping pills, can suppress certain stages of sleep. This can result in sleep, that is loss of consciousness, without fulfilling its physiological functions.
An organism that is prevented from returning to its sleep state soon has its basic life functions impaired. If the insufficiency is small, a sleep debt can accumulate, leading to drowsiness. Severe lack of sleep, up to and including a total lack of sleep, can have quite severe psychological effects. Randy Gardner set the record for human sleeplessness by going without sleep for up to eleven days; during the last part of his ordeal, he was effectively non-functional. However, he was able to fully recover after sleeping. Although there are occasional stories of people who are able to function with a small amount of sleep, these cases do not appear to hold up under controlled conditions.
Experiments with rats have been designed to measure the effects of severe sleep deprivation. In one, a pair of rats was placed on a platform, separated by a movable wall. Both were instrumented with electroencephalograms. Whenever the “subject” rat began to show signs of sleep, the partition was moved, forcing both rats to move. The “control” rat, however, could sleep in between movements. After several weeks of this, the subject rats became unable to regulate body temperature; even if they were allowed to sleep at this point, they died shortly afterward. As of 2004, it is as yet unclear exactly what the mechanism which causes death in rats due to sleep deprivation is. There does not appear to be any specific organ failure, and adminstering antibiotics and autopsies appears to rule out a general collapse of the immune system.
It is also unclear the degree to which the results of sleep deprivation in rats can be generalized to humans. There are no reported cases of a human being dying or suffering any permanent ill effects solely from sleep depreviation (although the loss of mental functioning due to lack of sleep has been cited as the indirect cause for many fatal accidents). Furthermore, while loss of sleep results in adverse psychological changes to human beings, experiments in humans do not show the physiological effects (such as loss of temperature regulation) that are seen in rats. Why this is so is a subject of some debate among sleep researchers, with some arguing that perhaps sleep performs some functions in rats that also exist in waking humans, or that humans have some protection mechanism that causes people to eventually fall asleep regardless of external stimulus.
A majority of sleep disorders which originate within the body (for example, insomnia, Delayed Sleep Phase Syndrome, or Advanced Sleep Phase Syndrome) result from errors in synchronization of sleep with the body clock. Only a fraction of sleep problems are organic and cannot be resolved with chronotherapy. One of the simplest solutions towards getting good sleep is free-running sleep. Free-running sleep entails ignoring alarm clocks and schedules in order to sleep when, and only when tired. Free-running sleep can resolve the majority of synchronization-dependent sleep disorders, but usually cannot be employed due to the resulting loss of synchronization of sleep with the outside world (including day-night cycle).
Sleep disorders are often observed in patients with a number of psychiatric problems (e.g. bipolar disorder, depression, schizophrenia, etc.).
One form of sleep disorder, narcolepsy, probably has a genetic basis. Subjects not only fall asleep several times during the day, they also experience abnormal sleep patterns at night. A new medication is Xyrem, the proprietary name of gamma-hydroxybutyrate (GHB). In the United States of America, it has recently been approved by the Food and Drug Administration. [Latest Sleep disorder in USA]
Sleep can also refer to the state of hypnosis.