PART D

Sleep is a Cyclical Process

During sleep, people experience repeated cycles of NREM and REM sleep, beginning with an NREM phase. This cycle lasts approximately 90 to 110 minutes and is repeated four to six times per night. As the night progresses, however, the amount of deep NREM sleep decreases and the amount of REM sleep increases. The term ultradian rhythm (that is, rhythm occurring within a period of less than 24 hours) is used to describe this cycling through sleep stages.