is a resting state for both the mind and body where most stimuli from the
external world are blocked. In
normal sleep, people have a decrease in body temperature, blood pressure,
rate of breathing and most other body functions. The brain, however,
never decreases in activity and remains as active during sleep as it does
when the person is awake.
During the night's sleep, a normal, healthy adult alternates between non-REM and REM (rapid eye movement - the dream state) sleep. Non-REM sleep makes up about 75% of the night's sleep. In non-REM sleep, there are 4 stages ranging from light to very deep sleep. A person normally cycles through these stages 4 to 5 times a night.
Stage 1 non-REM sleep is a transition period from wakefulness to sleep. It can be described as dozing. During this period muscles and breathing begin to relax and the individual falls into a light sleep from which he/she can be easily awakened.
Stage 2 comprises approximately 45% of non-REM sleep. This is considered the onset of real sleep. Eye movements stop and brain waves indicate that sleep has officially begun.
Approximately 12% of non-REM sleep is spent in stage 3. In this stage, sleep becomes progressively deeper and the sleeper becomes more dificult to arouse.
Stage 4 is characterized by very deep sleep. It makes up about 13% of non-REM sleep. If awakened during very deep sleep, a person will often feel groggy and disoriented for several minutes. The differences between Stage 3 and 4 are difficult to determine and depend upon interpretation of brain wave characteristics.
Non-REM sleep makes
up about 75% of the night's sleep. REM sleep comprises the other 25%. Most
dreaming takes place during REM sleep. REM sleep is very different from
non-REM sleep. During REM sleep there is periodic eye movement and eyelid
fluttering, muscle paralysis, and irregular breathing. Body temperature
falls and heart rate and blood pressure become irregular. During REM sleep,
the brain blocks muscle activity so that the dreams will not be acted out.
Sleep is a necessary biologic function. Without sufficient sleep, individuals become irritable and unable to concentrate. Work performance is likely to become impaired and there is excessive daytime drowsiness. It is estimated that 50% of the adult population of the United States is sleep deprived. Sleep deprivation is a significant factor in traffic and workplace accidents.