Sleep is the fundamental anabolic process common to all life forms, plant and animal. In animals, the sleeping state is characterized by an absolute minimal degree of consciousness and decreased responsiveness to the surrounding world.
Sleep is a dynamic, constructive time of healing and growth for animal organisms. The simple substances which have been ingested during the catabolic (awake) period are synthesized into the complex proteins of living tissue. The waking life of animal organisms is a dynamic, destructive time because the organisms' complex proteins are torn down and exhausted as they are used for activities including locating and ingesting preformed organic molecules to meet the immediate energy needs of the wakened state and to provide the building block proteins which fuel the anabolic dynamics that occur during sleep.
It may well be that plant organisms exist in a perpetual anabolic (sleep) state. No catabolic (wakened) state has ever been documented for any plant organism and, by definition, plants do not exhibit the characteristics of animal organisms in a wakened state.
Every animal organism's lifelong alternation between anabolic and catabolic periods, what we view as being asleep and being awake, is a cyclical pattern which occurs daily. The human inner body clock seems to be based on a 24.5-25.5 hour cycle, according to some studies.
The daily cycle of sleep and wakefulness is regulated by various hormones produced by the hypothalamus and external stimuli, the level of sunlight being the most obvious example. The body's levels of certain neurohormones are highly correlated with the sleep and wakened states. The levels of the orexin (hypocretin) neurohormone pair rise sharply to mark the transition from the anabolic, sleep state to the catabolic, wakened state. The melatonin level is high at times during the anabolic state. Adenosine, a nucleoside which plays various roles in biochemical processes, gradually accumulates in the human brain during wakefulness but decreases during sleep.
The anabolic processes which characterize the sleep state do not require any amount of consciousness. Sleep is very different from other forms of unconsciousness such as coma. During sleep an animal organism's organs, including the brain, remain very active. Sleep can always be interrupted if given enough external stimulus.
It is possible (indeed, normal) to sleep for short periods several times daily (afternoon nap, siesta, for example).
Stages of sleep
Studies of human sleep have established five well defined stages, according to electroencephalographic (EEG) recordings:
- 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
- Stage 5: Rapid Eye Movement (REM) sleep, associated with dreaming.
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.
The deep hypothalamus part of the brain is a part of the very oldest part of the brain, the Limbic system, from an evolutionary standpoint. It produces a pair of hormones called orexin into the brain, which wakes up animals. When the hypothalamus stops releasing this neurohormone animals fall asleep. Without it, animals would just sleep and never wake up.
Most efforts to understand sleep in the past have failed because they have focused exclusively upon sleep as a phenomenon which affects the human brain. These narrow approaches make little sense because sleep is universal among all animal life forms, including those which don't even have brains.
One of the major functions of sleep is consolidation and optimization of memories (including "unlearning"). This does not, however, explain why sleep is universal and essential. It also does not explain why mental functions are so grossly impaired by sleep deprivation and why sleep deprivation is lethal. We can begin to understand why sleep deprivation is lethal, however, if we accept sleep as a necessary time of anabolic activity among all animal organisms and the basic need for anabolic activity as a prerequisite for life itself.
The state of sleep is clearly an anabolic state, by definition, because it is marked by physiological processes of growth and rejuvenation of the organism's immune, nervous, muscular and skeletal systems. Sleep restores neurons and increases production of brain proteins and certain hormones. Wakefulness is a cyclical, 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 balanced metabolic processes (anabolism and catabolism), more familiar to us as the two balancing states of sleep and wakefulness.
Adequate rest and a properly functioning immune system are closely related. 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.
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.
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. Failure to sleep is characterized by progressively severe psychological and physical distress. Randy Gardner attempted to resist sleep in an uncontrolled 'experiment' in 1965. As his ordeal progressed he fell into a silent stupor, bringing into doubt whether he was actually awake in any practical sense. There are occasional stories of people who are able to function with a small amount of sleep but these cases do not appear to hold up under controlled conditions.
In October of 2002, the Associated Press reported that a man had spent four days and nights continuously playing video games, without sleeping, in a cybercafe in Kwanju , 260 kilometers south-west of Seoul and was found dead in the cybercafe’s toilet. 
A recent study by the Department of Psychiatry and Psychology at the University of Pittsburgh School of Medicine shows that poor sleep and sleep deprivation in older adults can lead to earlier death, but another survey of more than a million people in the 1980s found that those who slept more than seven and a half hour tended to die a little earlier.
Despite the risks, sleeping less is attractive because of additional time and many people feel they have to sleep less to cope with chosen lifestyle (e.g. because of extremely long work hours).
Modafinil can be used to reduce drowsiness and cope with shorter sleep. Other drugs (amphetamine, caffeine, methylphenidate) are less precise, not as effective or have greater side effects.
Polyphasic sleep is a method for minimizing the time spent asleep while maximizing the effects. This is done by sleeping in short naps throughout the day, minimizing the time spent awake between each period of sleep and thus decreasing the workload of the brain while sleeping.
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 administering antibiotics, and autopsies appears to rule out a general collapse of the immune system.
Some recent studies (late 2003/early 2004) not only concluded the cause of death to be more closely related to REM deprivation, also found the rats to die in about a week less time. It is believed this is because, unlike non-REM sleep that repairs parts of the brain damaged by metabolism and free radicals, REM sleep repairs the repair center. Despite the fact that rats die in this type of experiment, humans do not. 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 dying or suffering any permanent ill effects solely from sleep deprivation (although the impairment of mental functioning, i.e., hallucinations, associated with being awake for extended periods has been cited as the cause of 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. This supposition 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.
See sleep disorder for more information
A majority of sleep disorders which originate within the body (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, is tied to the hypothalamus' failure to produce the orexin hormone pair sufficiently for normal human life and may have 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.
Sleep can also refer to the state of hypnosis.
Last updated: 10-18-2005 08:49:03