NREM and REM sleep alternate cyclically throughout the night starting with NREM sleep lasting about 80 min followed by about 10 min of REM sleep. This 90-min sleep cycle is repeated three to six times each night. Each REM period typically ends with a brief arousal or transition into light NREM sleep.
In successive cycles the amount of NREM sleep decreases and the amount of REM sleep increases. SWS is usually confined to the first two sleep cycles. The diagrammatic representation of overnight sleep structure is known as a hypnogram, a simplified form of which is shown in Fig 1.
This conventional sleep staging is concerned with the macrostructure of sleep but there is increasing interest in the microstructural fragmentation of sleep by frequent, brief arousals (seen mainly in the EEG) lasting a matter of seconds without obvious clinical accompaniments. This subtle type of sleep disruption, overlooked by conventional sleep staging, is increasingly associated with impairment of daytime performance, mood, and behaviour, more so than any one stage of sleep. (9)
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