Two sleep states have been identified: rapid eye movement (REM) sleep, and non-REM (NREM) sleep. REM sleep is a state of heightened arousal characterized by uneven breathing, heart rate, and blood pressure; intense electrical brain activity; loss of muscle tone; and darting eye movements. NREM sleep is marked by reduced brain activity; regular patterns of heart rate, breathing, and blood pressure; and general body quiescence.
For older children and adults, NREM sleep can be further differentiated into four progressively deeper levels, with stages three and four (also called delta or slow-wave sleep) representing the deepest levels. This slow-wave activity emerges at about three to six months, peaks during early childhood, and then decreases during adolescence.
Because infant sleep patterns do not approximate adult patterns until well into the first year, infant sleep states are described differently. Infants slip easily back and forth among several states of arousal that include three awake states (crying, waking activity, and quiet alertness), a transitional state (drowsiness), and two sleeping states (active and quiet sleep). Quiet sleep resembles NREM sleep in adults, but might include occasional startle movements or sucking. Active sleep, although similar to adult REM sleep, is characterized by much movement of the limbs, as well as twitching, smiling, and rapid eye movements beneath closed or partially closed lids. Brain wave patterns are highly similar to awake patterns.
Sleeping patterns are cyclical and are controlled by two biological ''clocks.'' The first, which originates in the suprachiasmatic nucleus of the hypothalamus, controls daily cycles. Cycles develop before birth and can be detected in utero at about twenty weeks; as the clock matures, however, there is a gradual change from multiple cycles to a single daily pattern. The second clock, originating in the pons section of the brain stem, regulates the alternation between REM and NREM sleep. Here, too, maturational changes can be seen. Newborns typically fall first into REM sleep, whereas older children may not experience REM for three hours and the adult pattern (about ninety minutes after falling asleep) is not established until adolescence.
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