Sleep is a state of physical inactivity and mental rest in which conscious awareness, thought, and voluntary movement cease and intermittent dreaming takes place. This natural and regular phenomenon essential to all living creatures normally happens with the eyes closed and is divided into two basic types:REM (rapid eye movement) and NREM (non-rapid eye movement) sleep. As passiveas sleep appears, it is actually a very active and deliberate process in which the brain busily turns off wakeful functions while turning on sleep mechanisms. No one knows exactly why we must sleep or how it happens, but the quality, quantity, and type of sleep affects the quality, quantity, and effectiveness of our wakeful mental and physical activities. These, in turn, influence the quality, quantity, and timing of sleep.

In the attempt to understand our need for sleep, experiments in sleep deprivation play an important role. Total sleep deprivation longer than 40 hours proves impossible, however, due to brief, totally unpreventable periods of "microsleep," which will happen even during physical activity. These microsleeps barely last a few seconds, but they may explain performance lapses in waking activities. They demonstrate the body's obvious need for sleep and may even have some restorative function.

While sleep deprivation can eventually cause death, sleep deprivation lastingup to 10 days shows no serious, prolonged consequences and does not cause severe psychological problems or mental illness as once thought. Losing more than one night's sleep does produce a noticeable increase in irritability, lethargy, disinterest, and even paranoia. While not seriously impaired, psychomotor performance and concentration are adversely affected. While autonomic (involuntary) nervous system activity increases during sleep deprivation to keepheart rate, blood pressure, breathing, and body temperature normal, physicalfitness cannot be maintained and immunological functions seem to suffer.

Another question that remains only partially answered is how sleep onset is determined and why. The factors involved include circadian rhythms (biologicaltime clocks); the degree of stimulation in the wakeful state; the degree ofpersonal sleepiness; the decrease in core body temperature; a quiet and comfortable sleep environment; conditioning arising from "bedroom cues"; and homeostasis, the automatic attempt by the body to maintain balance and equilibrium(for example, the air temperature may fall to 50°F [10°C], but our body burns calories to maintain its normal temperature of 98.6°F [37°C]).

The fact that sleep deprivation increases the desire for sleep firmly pointsto a homeostatic element in sleep. This is intricately linked to highly influential circadian rhythms controlled by centers probably located in the hypothalamus, part of the brain primarily involved in autonomic nervous system functions. Circadian rhythms determine our approximate 24- to 25-hour sleep-wakepattern and a similar cycle in the rise and fall of core body temperature andother physiological functions.

Studies done on human circadian rhythms in situations totally devoid of timecues (such as sunrise, sunset, clocks, etc.) show that these rhythms are controlled completely internally and usually run on a cycle of almost 25 rather than 24 hours. In normal situations, factors called "zeitgebers" (from the German zeit for time and geber for giver) such as daylight, environmental noises, clocks, and work schedules virtually force us to maintain a 24-hour cycle. Therefore, our circadian rhythms must "phase advance" from their normal, approximate 25-hour cycle to an imposed 24-hour cycle.

The body has difficulty adapting to much more than an hour of phase-advance in one day. Drastic time changes--like those caused by rapid long-distance travel such as flying--require either phase-advancement or phase-delay. This iswhy travelers experience "jet lag." Recovery from east-west travel requiringphase-delay adjustments is usually quicker than in phase-advancement resulting from west-east travel. Some people seem simply unable to phase-advance their biological clocks, which often results in sleep disorders.

The greatest contribution to sleep study was the development of the EEG, or electroencephalogram, by German psychiatrist Hans Berger in 1929. This electrode, attached to the scalp with glue, records electrical impulses in the braincalled brain waves. The discovery triggered investigations into sleep in major centers around the world. Specific brain wave patterns became evident andsleep was generally classified into distinct stages.

In 1953, Professor Nathanial Kleitman and his graduate student Eugene Aserinsky reported their close observations of a sleep stage they called REM--rapideye movement. An electro-oculogram, or EOG, taped close to the eyelids, recorded both vertical and horizontal eye movement, which became rapid and sporadic during REM sleep. The electromyogram, or EMG, recorded chin and neck musclemovement, which, for as yet undetermined reasons, completely relaxed duringREM sleep. Kleitman and Aserinsky found that when subjects were awakened fromREM sleep they almost always reported a dream, which was seldom the case when awakened from non-REM sleep.

Following the initial REM discoveries, sleep research greatly increased. Oneimportant discovery arising from this research was the high prevalence of sleep disorders, some of which now explain problems previously blamed on obscurephysical or psychological disorders but which could not be effectively treated by medicine or psychiatry.

Non-REM sleep is generally believed to occur in four stages and is characterized by lack of dreaming. In the drowsy, light sleep of stage 1, which takes up about 5% of the sleep cycle, the sleeper is generally nonresponsive but iseasily awakened. High chin muscle activity occurs and there is occasional slow, rolling eye movement.

Within a few minutes, the sleeper enters stage-2 sleep. Brain waves slow further and spindles (short bursts of electrical impulses) appear, along with K-complexes (sharp, high voltage wave groups, often followed by spindles). Thesephenomenon may be initiated by internal or external stimuli or by some as yet unknown source deep within the brain. This portion of sleep occupies about45% of the sleep cycle.

Normally, stage-3 sleep follows stage 2 as a short (about 7% of total sleep)transition to stage-4 sleep. There is virtually no eye movement during stages2, 3, and 4.

In stage-4 sleep, some sleep spindles may occur, but are difficult to record.This stage occupies about 13% of the sleep cycle, seems to be affected morethan any other stage by the length of prior wakefulness, and reflects the most cerebral "shutdown." Accordingly, some researchers believe this stage to bethe most necessary for brain tissue restoration. Usually grouped together, stages 3 and 4 are called delta, or slow wave sleep (SWS), and is normally followed by REM sleep.

The sleep cycle from stage 1 through REM occurs three to five times a night in a normal young adult. Stages 3 and 4 decrease with each cycle, while stage2 and REM sleep occupy most of the last half of the night's sleep. Time spentin each stage varies with age, and age particularly influences the amount time spent in SWS. From infancy to young adult, SWS occupies about 20-25% of total sleep time and perhaps as little as 5% by the age of 60. This loss of time is made up in stage-1 sleep and wakeful periods.

The period comprised of the four stages between sleep onset and REM is knownas REM latency. REM onset is indicated by a drop in amplitude and rise in frequency of brain waves. The subject's eyes flicker quickly under the eyelids,dream activity is high, and the body seems to become paralyzed because of thedecrease in skeletal muscle tone. After REM, the subject usually returns tostage 2 sleep, sometimes after waking slightly. REM sleep occurs regularly during the night. The larger the brain, the longer the period between REM episodes--about 90 minutes for humans and 12 minutes in rats.

REM sleep is triggered by neural functions deep within the brain, which release one type of neurotransmitter (chemical agent) to turn REM sleep on and another to turn it off. Whereas autonomic activity (such as breathing and heartrate) slows and becomes more regular during non-REM sleep, it becomes highlyirregular during REM sleep. Changes in blood pressure, heart rate, and breathing regularity take place, there is virtually no regulation of body temperature, and clitoral and penile erections are often reported. Most deaths, particularly of ill or aged individuals, happen early in the morning when body temperature is at its lowest and the likelihood of REM sleep is highest.

REM activity is seen in the fetus as early as six months after conception. Bythe time of birth, the fetus will spend 90% of its sleep time in REM but only about half that after birth. REM constitutes about 20-30% of a normal youngadult's sleep, decreasing with age. These observations support one of several theories about our need for REM sleep which suggests that, to function properly, the central nervous system requires considerable stimulation, particularly during development. Because it receives no environmental stimulation during the long hours of sleep, it is possible that the high amount of brain waveactivity in REM sleep provides the necessary stimulation.

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