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.
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.
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.
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 Testing).
Last full review/revision January 2013 by Karl Doghramji, MD