what are the sleep cycles
Human sleep occurs in periods of approximately 90 minutes, which include increasing proportions of paradoxical (REM) sleep as they repeat. This rhythm is called the ultradian sleep cycle. Sleep proceeds in cycles of NREM and REM, normally in that order and 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.