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A periodic state of rest accompanied by varying degrees of unconsciousness and relative inactivity. Sleep is thought of as something that occurs once each 24-hr day, but at least half of the world's population has an afternoon nap or siesta as part of their lifelong sleep-wake pattern. The need for and value of sleep is obvious; yet there is no explanation of why it provides a daily renewal of a feeling of health and well-being.
The sleep-wake cycle varies in relation to the age and gender of the individual. The newborn may sleep as much as 20 hr each day; a child, 8-14 hr, depending on age; adults, 3-12 hr with a mean of 7-8 hr, which may decrease to 6.5 hr in older adults. Women over 35 tend to sleep more than men. There is great individual variation in the amount and depth of sleep.

Sleep has two states: non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. NREM sleep and REM sleep alternate during the night; each cycle requires 90-100 min. NREM sleep makes up approx. 75% of the sleep cycle, and REM sleep approx. 25%, with individual variations.

People deprived of sleep for several days or more become irritable, fatigued, unable to concentrate, and, usually, disoriented. Performance of mental and physical tasks deteriorates. Some people experience paranoid thoughts and auditory, visual, and tactile illusions or hallucinations. Deprivation of REM sleep may cause anxiety, overeating, and hypersexuality. The effects of sleep deprivation are reversed when the normal sleep-wake cycle is resumed.
SEE: non-rapid eye movement sleep; SEE: rapid eye movement sleep

The following physiological changes occur during sleep: body temperature falls; secretion of urine decreases; heart rate and respiration become slower and more regular during NREM sleep and then more rapid and less regular during REM sleep. During REM sleep, blood flow to the brain is increased; breathing is more irregular; heart rate and blood pressure vary; cerebral blood flow and metabolic rate increase; and penile erections may occur. There is an increased secretion of growth hormone during the first 2 hr of sleep; surges of adrenocorticotropic hormone (ACTH) and cortisol secretion occur in the last half of the sleep period. Luteinizing hormone secretion is increased during sleep in pubescent boys and girls, and prolactin secretion is increased in men and women, esp. immediately after the onset of sleep.

In evaluating sleep, it is important to know that hand waving, arm swinging, laughing, and flatus occur during normal sleep. Snoring may be clinically insignificant but, when accompanied by apnea, can be harmful.

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