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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