![]() ![]() Within 90 minutes after sleep begins, an adult progresses through all four stages of NREM sleep and then proceeds into the first of a series of REM periods of sleep. NREM sleep is increased after physical activity and has a relatively high priority among humans in the recovery sleep following extended periods of wakefulness. The EEG patterns of NREM sleep suggest that this is the kind of apparently restful state that supports the recuperative functions assigned to sleep. ![]() Stages 3 and 4 consist of relatively high voltage EEG tracings with a predominance of delta wave activity (one to two hertz). Stage 2 is characterized by intermittent waves of 12 to 16 hertz, known as sleep spindles. Stage 1 is observed immediately after sleep begins or after momentary arousals and is characterized by low-voltage, mixed-frequency EEG tracing, with predominantly theta-wave activity (four to seven hertz, that is, cycles per second). ![]() On the basis of electroencephalographic (EEG) criteria, NREM sleep is subdivided into four stages. Studies concerned with the measurement of central and autonomic activities during sleep have led to its division into two types: non–rapid eye movement (NREM) sleep, also called orthodox or synchronized (S) sleep and rapid eye movement (REM) sleep (so called because of the rapid eye movements during this stage), also called paradoxical or desynchronized (D) sleep. Prior to the discovery and reporting of rapid eye movements during sleep, it was thought that sleep was a single state of passive recuperation in which the central nervous system was deactivated. ![]()
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