A typical sleep cycle consists of NREM 1 followed by
NREM 2, 3, and 4 with possible drifting back through previous
stages of NREM 3 and 2 before the REM stage
starts. REM sleep is followed by more Stage 2 NREM.
The cycle, usually about 90 minutes long, repeats itself
throughout a full night’s sleep. Awakenings in the middle
of the night alter the normal progression of sleep with a return
to Stage 1 NREM and disrupt the normal sleep cycle.
A number of mechanisms regulate sleep. Chief among
them is the circadian drive for sleep and wakefulness. An intrinsic
body clock residing in the suprachiasmatic nucleus
within the hypothalamus regulates a complex series of
rhythms in humans, including the circadian drive for sleep