Overview of Sleep
Sleep is necessary for survival and good health, but why sleep is needed and exactly how it benefits people are not fully understood. One of sleep's benefits is its restorative effect on people's ability to function normally during the daytime. Sleep may be restorative because during sleep, the body removes a toxic protein called beta-amyloid (a waste product of the brain's normal activity) from the brain. When this protein accumulates, it interferes with the brain's functioning.
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.
How long people sleep and how rested they feel after waking can be influenced by many factors, including the following:
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.
Snoring may interfere with sleep—the snorer's or the snorer's bed partner's.
All sleep is not the same. There are two main types of sleep:
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. People perceive stage 3 as high-quality sleep.
REM sleep: Electrical activity in the brain is unusually high, somewhat resembling that during wakefulness. The eyes move rapidly, and certain 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
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
People with insomnia have difficulty falling and staying asleep and wake up feeling unrefreshed. They may wake up early. 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. Unusual movements and behaviors during sleep are called parasomnias.
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.