Stage 1 sleep, or drowsiness, is often described as first in the sequence, especially in models where waking is not included.
Polysomnography shows a 50 percent reduction in activity between wakefulness and stage 1 sleep.
Since the early 20th century, human sleep has been described as a succession of five recurring stages: four non-REM stages and the REM stage.
(A sixth stagerelaxed wakefulness, the phase during which a person falls asleepis sometimes included).
REM sleep is distinguishable from NREM sleep by changes in physiological states, including its characteristic rapid eye movements.
However, polysomnograms show wave patterns in REM to be similar to Stage 1 sleep.So, a normal sleep cycle has this pattern: waking, stage 1, 2, 3, 4, 3, 2, REM.Usually, REM sleep occurs 90 minutes after sleep onset.The period of non-REM sleep (NREM) is comprised of Stages 1–4 and lasts from 90 to 120 minutes, each stage lasting anywhere from 5 to 15 minutes.Surprisingly, however, Stages 2 and 3 repeat backwards before REM sleep is attained.The first period of REM typically lasts 10 minutes, with each recurring REM stage lengthening, and the final one lasting an hour.The five stages of sleep, including their repetition, occur cyclically.Sleep cycles subsequent to the first one in a night's sleep typically feature less slow-wave sleep, as Stages 3 and 4 shorten.Slow-wave, deep sleep is longest early in a night's sleep.So a person may complete five cycles in a typical night's sleep.The sleep cycle is variable, influenced by several agents.