Observation and electrophysiological (EEG) examination of sleep have led to the recognition of two distinct phases of sleep: slow-wave or non-rapid eye movement (NREM) sleep (approximately 80 per cent of total sleep time), and rapid eye movement (REM) sleep (20 per cent of total sleep time). Following a period of 10 to 20 min of sleep latency, individuals enter NREM sleep which is divided into four stages, each characterized by its EEG pattern. Stage I NREM, which accounts for 10 per cent of NREM sleep, is identified by a decrease in a (awake) activity and the appearance of both rhythms (4-7 Hz) and b rhythms (above 13 Hz). Stage II takes up about 50 per cent of NREM sleep and is characterized by the appearance of K spindles and sleep spindles (12-14 Hz) with less than 20 per cent of activity (below 4 Hz). Stages III and IV are marked by progressive increases of activity and represent deeper and more restful sleep. REM sleep occurs 60 to 90 min after the onset of sleep and recurs at 90-min intervals throughout the night. It is more frequent and of greater duration towards the waking hours. EEG findings include desynchronization with low-voltage fast-wave activity and variable amounts of activity and 'sawtooth' waves. Two phases of REM sleep are recognized. The tonic phase, which lasts throughout REM sleep, is marked by hypotonia of the neck, facial, and antigravity muscles. Superimposed on the tonic phase are phasic events during which rapid eye movements occur and myoclonic twitching of the facial muscles is seen.
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