Stages Human sleep occurs in periods of approximately 90 minutes, which include an increasing proportion of paradoxical (REM) sleep as they repeat. This rhythm is called the ultradian sleep cycle. Sleep proceeds in cycles of REM and NREM, usually four or five of them per night. The American Academy of Sleep Medicine(AASM) divides NREM into three stages: N1, N2, and N3, the last of which is also called delta sleep or slow-wave sleep.
The whole period normally proceeds in the order: N1 → N2 → N3 → N2 → REM. There is a greater amount of deep sleep (stage N3) earlier in the night, while the proportion of REM sleep increases in the two cycles just before natural awakening. Each stage may have a distinct physiological function and this can result in sleep that exhibits loss of consciousness but does not fulfill its physiological functions (i.e., one may still feel tired after apparently…
