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Overview of Sleep

by Karl Doghramji, MD

Sleep is necessary for survival and good health, but why sleep is needed and exactly how it benefits people are not fully understood. Individual requirements for sleep vary widely, usually from 6 to 10 hours every day. Most people sleep at night. However, many people must sleep during the day to accommodate work schedules—a situation that can lead to sleep disorders.

Did You Know...

  • No one knows exactly why people need to sleep.

How long people sleep and how rested they feel after waking can be influenced by many factors, including level of excitement or emotional distress, age, diet, use of drugs. and aging. For example, some drugs make people sleepy, and others make sleeping difficult. Some food components or additives, such as caffeine, strong spices, and monosodium glutamate (MSG), may disturb sleep. Older people tend to fall asleep earlier, to awaken earlier, and to be less tolerant of changes in sleep habits.

The sleep cycle

All sleep is not the same. There are two main types of sleep:rapid eye movement (REM) sleep and nonrapid eye movement (non-REM) sleep, which has three stages. People normally cycle through the three stages of non-REM sleep (stages N1 through N3), usually followed by a brief interval of REM sleep, every 90 to 120 minutes or several times every night. Throughout the night, people wake up briefly (called stage W) but are typically unaware of being awake.

  • Non-REM sleep: Non-REM sleep accounts for about 75 to 80% of total sleep time in adults. Sleep progresses from stage 1 (the lightest level, when the sleeper can be awakened easily) to stage 3 (the deepest level, when the sleeper can be awakened with greater difficulty). In stage 3, blood pressure is at its lowest, and heart and breathing rates are at their slowest.

  • REM sleep: Electrical activity in the brain is unusually high, somewhat resembling that during wakefulness. The eyes move rapidly, and muscles are paralyzed so that voluntary movement is impossible. However, some muscles may twitch involuntarily. The rate and depth of breathing increase.

The most vivid dreaming occurs during REM sleep. Most talking during sleep, night terrors, and sleepwalking occur during stage 3, which is non-REM sleep.

Stages of the Sleep Cycle

People normally cycle through distinct stages of sleep every 90 to 120 minutes during the night: three stages of nonrapid eye movement sleep (N) and one stage of rapid eye movement (REM) sleep. Relatively little time is spent in stage N1 (shallow) sleep. The greatest time is spent in stage N2 sleep. Deep sleep (stage N3) occurs mostly during the first half of the night. More time is spent in REM sleep as the night progresses. Brief awakenings (called stage W) occur throughout the night, but the sleeper is typically unaware of most of them.

Sleep disorders

Sleep disorders involve disturbances in the ability to fall asleep, stay asleep, or stay awake or unusual behaviors during sleep, such as sleepwalking. Sleep can be disturbed by many factors, including irregular bedtimes, activities before bed, stress, diet, disorders, and drugs.

The most common symptoms of sleep disorders are insomnia and excessive sleepiness during the day. People with insomnia have difficulty falling and staying asleep and wake up feeling unrefreshed. Lack of sleep makes people feel sleepy, tired, and irritable during the day. People with excessive daytime sleepiness tend to fall asleep during normal waking hours. Some sleep disorders make people unable to resist falling asleep during the day.

Some sleep disorders involve involuntary movements of the limbs or other unusual behaviors (such as nightmares, night terrors, or sleepwalking) during sleep.

Other symptoms may include problems with memory, coordination, and emotions. People may perform less well in school or at their jobs. The risk of having a motor vehicle accident or developing a heart disorder is increased.

A detailed description of the problem, sometimes with information from a sleep log, usually indicates the diagnosis, but sometimes testing in a sleep laboratory is needed. This testing includes polysomnography (see Sleep Disorders:Testing).

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