Organization of sleep
The amount of sleep obtained during the night varies among individuals; most adults need about 7-9 hours of sleep per night. In addition to genetic factors that influence daily sleep needs, age and medical or psychiatric disorders also strongly influence sleep patterns.
Regardless of the number of hours needed, the proportion of time spent in each stage and the pattern of stages across the night is fairly consistent in normal adults.
A healthy young adult will typically spend about 5% of the sleep period in stage 1 sleep, about 50% in stage 2, and 20-25% in each of SWS (stages 3 and 4) and REM sleep. Sleep occurs in cycles of NREM-REM sleep, each lasting approximately 90 to 110 minutes.
SWS (stages 3 and 4) is most prominent early in the night, especially during the first NREM period, and diminishes as the night progresses. As SWS wanes, periods of REM sleep lengthen, while showing greater phasic activity and generally more intense dreaming later in the night.