The Cleveland Clinic

Sleep Disorders: Sleep 101

Introduction to sleep

When you sleep, your body rests and restores its energy levels. However, sleep is an active state that affects both your physical and mental well-being. A good night's sleep is often the best way to help you cope with stress, solve problems, or recover from illness.

What Happens During Sleep?

Sleep is prompted by natural cycles of activity in the brain and consists of two basic states: Rapid Eye Movement (REM) sleep and Non-REM (NREM) sleep, which consists of Stages 1 through 4.

During sleep, the body cycles between non-REM and REM sleep. Typically, people begin the sleep cycle with a period of non-REM sleep followed by a very short period of REM sleep. Dreams can occur in any stage of sleep.

What Is Non-REM Sleep?

The period of NREM sleep is made up of stages 1-4. Each stage can last from 5 to 15 minutes. Stages 2 and 3 repeat backwards before REM sleep is attained.

  • Stage 1: Polysomnography (sleep readings) shows a 50% reduction in activity between wakefulness and stage 1 sleep. The eyes are closed during Stage 1 sleep. However, if aroused from this stage of sleep, a person may feel as if he or she has not slept. Stage 1 may last for five to 10 minutes.
  • Stage 2: This is a period of light sleep during which polysomnographic readings show intermittent peaks and valleys, or positive and negative waves. These waves indicate spontaneous periods of muscle tone mixed with periods of muscle relaxation. The heart rate slows and the body temperature decreases. At this point, the body prepares to enter deep sleep.
  • Stages 3 and 4: These are deep sleep stages, with stage 4 being more intense than Stage 3. These stages are known as slow-wave, or delta, sleep. During NREM sleep, the body repairs and regenerates tissues, builds bone and muscle, and appears to strengthen the immune system. As you get older, you get less NREM sleep. People under age 30 have about two hours of restorative sleep every night, while those over 65 may get only 30 minutes.

What Is REM Sleep?

Usually, REM sleep occurs 90 minutes after sleep onset. The first period of REM typically lasts 10 minutes, with each recurring REM stage lengthening, and the final one lasting an hour. Polysomnograms show brainwave patterns in REM to be similar to stage 1 NREM sleep. In people without sleep disorders, heart rate and respiration speed up and become erratic during REM sleep. The face, fingers, and legs may twitch.

Intense dreaming occurs during REM sleep as a result of heightened cerebral activity, but paralysis occurs simultaneously in the major voluntary muscle groups. REM is a mixture of encephalic (brain) states of excitement and muscular immobility. For this reason, it is sometimes called paradoxical sleep.

The percentage of REM sleep is highest during infancy and early childhood. During adolescence and young adulthood, the percentage of REM sleep declines, and the percentage decreases further in older age. In many cases, older people enter REM sleep more quickly and remain in REM sleep longer.

How Much Sleep Do You Need?

The amount of sleep a person needs depends on the individual. The need for sleep depends on various factors, one of which is age. Infants usually require about 16-18 hours of sleep per day, while teenagers need about 9 hours per day on average. Most adults need about 7-8 hours of sleep per day.

The amount of sleep a person needs also increases if he or she has been deprived of sleep. People do not seem to adapt to getting less sleep than they need.

What Are the Consequences of Too Little Sleep?

Too little sleep may cause:

  • Impaired memory and thought processes
  • Depression
  • Decreased immune response

Sleep deprivation also magnifies alcohols effects on the body, so a fatigued person who drinks will become much more impaired than someone who is well-rested. Caffeine and other stimulants can temporarily overcome the effects of severe sleep deprivation, but cannot do so for extended periods of time.

Reviewed by The Sleep Medicine Center at The Cleveland Clinic.




Edited by Michael J. Breus, PhD, WebMD, September 2004.

Portions of this page © The Cleveland Clinic 2000-2005


Last Editorial Review: 6/20/2005




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