Siegel, J.M. Sleep. In: Encarta Encyclopedia, 1999-present.


Brain Activity During Sleep Sleep, natural state of rest characterized by reduced body movement and decreased awareness of surroundings. Sleep is distinguished from other sleeplike states, for instance, hibernation or coma, because it is easily interrupted by external stimulation, such as a loud noise. While the exact purpose of sleep remains a mystery, sleep researchers have made enormous strides in understanding how sleep occurs in humans and other animals, and the nature of sleep disorders.

Bottle-nosed Dolphins All mammals and birds sleep, but scientists are unsure if reptiles, fish, insects, and other life forms sleep. Total sleep amounts differ greatly across species. In general, large mammals tend to sleep less than small mammals. The giraffe and elephant, for instance, sleep only 2 to 4 hours a day, while bats, opossums, and armadillos sleep 18 hours a day or more.

While sleeping, most animals close their eyes and adopt particular positions referred to as sleep postures. Humans typically lie down to sleep, for example, while giraffes kneel and bend their long necks around to rest their heads in the crook of their hind knee. Some animals, such as dolphins, can sleep while they are moving.

Scientists measure sleep by placing metal electrodes on the scalp to record the electrical activity of the brain. This procedure, called electroencephalography (EEG), enables sleep researchers to evaluate levels of brain activity at different times during sleep. Researchers use similar electrodes to record a sleeping person's body muscle activity and rate of eye movement.

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In the 1950s American physiologists Eugene Aserinsky and Nathaniel Kleitman reported that periods of eye movement and twitching occur during sleep. They named these periods rapid eye movement (REM) sleep. Aserinsky and Kleitman found that when subjects were awakened during REM sleep, they reported vivid dreams. Scientists believe that REM sleep is closely related to wakefulness because brain wave activity during REM sleep is marked by short, rapid wave patterns similar to brain wave activity of the waking state.

Sleep characterized by little or no eye movement is called nonrapid eye movement (NREM) sleep. During NREM sleep, breathing and heart rates slow down, and body temperature and blood pressure often decrease. When awakened from periods of NREM sleep, subjects are much less likely to report vivid, action-packed dreams. Brain wave activity during NREM sleep is dominated by large, slow waves that contrast markedly to the short, rapid wave patterns characteristic of REM sleep and the waking state.

Sleep studies based on EEGs have shown that during a normal night, humans cycle between REM sleep and NREM sleep in very regular patterns. In adults aged 20 to 60, REM sleep occurs about every 90 minutes. In this 90-minute cycle, humans fall into progressively deeper stages of NREM sleep, then cycle back through the stages until they enter REM sleep, and then the cycle begins again. In a normal night, the number of REM periods varies from four to six, depending on the length of the episodes and the total time asleep. REM episodes in the beginning of the night usually last about ten minutes and, during the night, grow progressively longer, lasting up to 30 minutes in the early hours of the morning. Most adults spend about 20 percent of their total sleep time in REM sleep.

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Sleep research shows that certain regions of the brain play critical roles in sleep. The brainstem, the portion of the brain just above the spinal cord, is critical in REM sleep control, while the forebrain is particularly important in NREM sleep control.

REM sleep is generated by a region in the brainstem, called the pons, and adjacent portions of the midbrain. Researchers have found that chemical stimulation of the pons will induce very long periods of REM sleep, while damage or injury to this brain region can greatly reduce or even prevent REM sleep. Animal studies have found that some neurons within the pons and midbrain are active only in REM sleep while other neurons in this region are entirely inactive only during REM sleep. Together, these neurons control muscle tone and other aspects of REM sleep. In REM sleep, most muscles in the body are turned off. This lack of muscle tone, called atonia, is particularly complete in the muscles of the back, neck, arms, and legs. Less affected are the muscles that move the eyes and the muscles responsible for breathing.

The combined effect of the sleep-active and sleep-inactive neurons explains why sleepers do not physically act out the vivid dreams they have during REM sleep and instead only twitch or make small movements. Humans with malfunctioning REM sleep-active and REM sleep-inactive systems thrash around in their sleep, often punching their bedmates or hurting themselves as they act out their dreams.

The neurons most critical to NREM sleep control are in the basal forebrain, the region of the brain lying in front of the hypothalamus. Researchers have found that people who have suffered damage or injury to the neurons in the basal forebrain have difficulty falling and staying asleep. Animal studies have shown that this area contains neurons that become most active before and during sleep. Many of these neurons are activated by heat, which explains how a warm bath or a hot day at the beach causes sleepiness.

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Mammal Sleep Requirements ( table ) A mounts of sleep vary significantly with age and even between individuals. Newborns sleep the most—a newborn baby sleeps between 17 and 18 hours a day, spending nearly half of that time in REM sleep. Both REM and NREM sleep decrease with age, and by age five, children sleep between 10 and 12 hours a day, spending about 20 percent of that time in REM sleep.

Sleep Requirements The average young adult seems to need about 8 hours of sleep per night to function optimally during waking hours. Some people, however, sleep just 6 or 7 hours a night, while others need more than 9 hours to feel rested. The elderly spend less time in deep NREM sleep, and their sleep is more easily interrupted.

REM sleep amounts also vary across animal species, depending on the size of the animal and its level of development at birth. The size of an animal seems to affect the type of sleep it experiences—small animals tend to spend more time in REM sleep. Animals that are born in relatively helpless states, such as opossums and humans, generally have more REM sleep as newborns than animals that can hunt, eat, keep warm, and defend themselves soon after birth, for instance guinea pigs or horses. Even as animals age into adulthood, those born relatively immature continue to spend more time in REM sleep than animals that are mature at birth.

One of the myths about sleep is that smarter animals spend longer periods in REM sleep. REM sleep amounts in humans or primates—believed to be the most intelligent members of the animal kingdom—are not remarkably high or low. Rather, they seem to fit the general rule of level of maturity at birth. REM sleep amounts in whales and dolphins—animals also recognized for their high intelligence—are among the lowest seen in any mammal. These animals, both born relatively mature, also seem to fit the general rule relating REM sleep amounts to level of maturity at birth.

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Although no one knows for sure why we sleep, there are a number of theories. Sleep may have evolved to protect animals from their predators by reducing their activity during the times when they are most vulnerable.

Research has shown that REM and NREM sleep may serve specific biological functions. Sleep deprivation studies reveal that humans and other animals respond to sleep loss in the same way. When study subjects are deprived of REM sleep, they tend to spend longer periods in REM sleep during their next sleeping period to make up for the loss. REM sleep after deprivation is more intense, with more eye movements per minute than in normal REM sleep. Similarly, subjects deprived of NREM sleep usually spend more time in NREM sleep afterward. EEGs measuring brain activity show that this rebound NREM sleep also differs from normal NREM sleep. This research suggests that the body needs adequate levels of both REM and NREM sleep. This conclusion has led many sleep researchers to believe that the two kinds of sleep serve different biological purposes, although the exact functions remain unclear.

The relationship between maturity at birth and REM sleep suggests that REM sleep plays a role in the development of the brain. REM sleep may have a related function later in life as well. However, that function remains a mystery.

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Sleep Apnea Snoring ( audio ) People who suffer from the most common sleep disorder, insomnia, have difficulty falling or staying asleep. Sleepiness caused by insomnia reduces concentration and slows reaction time during waking hours, leading to reduced productivity and accidents. One in three adults experiences some degree of insomnia at one time or another, especially during periods of stress. Longer-lasting cases of insomnia, called chronic insomnia, are less common and may be caused by a number of factors in addition to stress, including imbalances in body chemistry or other medical conditions.

Taken under the guidance of a qualified physician, sleeping pills are an effective treatment for temporary insomnia and may help some chronic insomnia patients. In some cases of insomnia, psychological treatments and physical exercise programs help patients identify or manage stress, enabling them to sleep better. Recent work has suggested that melatonin, a hormone naturally released into the bloodstream during the hours of darkness, may play a role in synchronizing sleep to a 24-hour cycle. Results from studies of the effects of melatonin on human sleep have been inconsistent. However, some evidence suggests that supplements of this hormone combat insomnia in older people who are melatonin deficient.

Narcolepsy is a sleep disorder that affects both NREM sleep and REM sleep mechanisms. Narcoleptics are persistently sleepy and experience periodic losses of muscle tone called cataplexy. During cataplectic attacks, the narcoleptic's muscles weaken, and if the attack is severe, the narcoleptic falls to the ground. Cataplectic attacks are triggered by sudden strong emotional reactions, such as laughter. A related symptom, called sleep paralysis, can occur when the narcoleptic is lying down, prior to falling asleep or just after awakening. At these times the person may lose muscle tone, resulting in an apparent paralysis, while remaining fully awake. Sleep paralysis can be terrifying if the narcoleptic does not realize that it is not life threatening. Animal studies have shown that the loss of muscle tone experienced by narcoleptics in waking results from an activation of the REM sleep-active and an inactivation of the REM sleep-inactive systems that normally function to reduce muscle tone in REM sleep. Narcoleptics are treated with stimulants to block sleepiness and with REM sleep suppressants to block cataplexy.

Children often experience one of several sleep disorders. Nocturnal enuresis, commonly known as bedwetting, typically occurs during NREM sleep. Sleep talking and sleepwalking also usually occur during NREM sleep (see  Somnambulism). Night terrors, typically marked by a scream and a terrified, confused awakening, affect many children. These disorders do not indicate any serious neurological or behavioral problem. Children suffering from them usually outgrow them by puberty, although sleep talking and sleepwalking may persist into adulthood.

In another common disorder, sleep apnea, relaxation of the muscles of the tongue and the soft palate at the base of the throat, allows the breathing passage to collapse in individuals with a narrow airway. Although chest movements may continue, no air flows into the lungs and oxygen levels in the blood decrease. When blood oxygen levels fall too low, the person briefly wakes to take a breath. This gasping breath can produce a loud, characteristic snort. The cycle of sleeping, airway collapsing, waking, and sleeping repeats, often hundreds of times in a night. Individuals with sleep apnea do not remember these brief awakenings and believe they slept through the night. However, the interrupted sleep leaves the individual exhausted in the morning and sleepy throughout the day. If left untreated, sleep apnea may also cause cardiovascular problems and greatly shorten life span. Effective treatments are available at medical centers specializing in sleep disorders. One treatment, called continuous-positive-airway-pressure (CPAP), uses a mask to deliver a stream of air through the nose, preventing airway collapse, restoring normal sleep. Sometimes surgical treatments that enlarge the airway can be effective.

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Contributed By: Jerome M. Siegel, B.S., Ph.D.
Professor, Department of Psychiatry, University of California, Los Angeles; Chief of Neurobiology Research, Sepulveda Veterans Affairs Medical Center, Los Angeles. Associate Editor, Sleep.

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