What are the Sleep Cycles?
Sleep is divided into 90-minute cycles of rapid eye movement (REM) and non-rapid eye movement (NREM) sleep; these 90-minute cycles are repeated three to six times during the night.
The time between the onset of sleep and the end of the first REM period defines the first sleep cycle. After that, sleep cycles (i.e., second through sixth cycle) always start with NREM sleep and end in REM sleep. (See Figure 1.2.)
The majority of slow-wave sleep (SWS) occurs in the first third of the night, while the majority of REM (and most dreams) occurs in the last third of the night.
Figure 1.2: Typical sleep pattern of a young human adult.1
Normal adults sleep an average of 7.5 hours per night, but this varies from about 6 to 9 hours per night; more extreme values of between 4 and 10 hours have been reported in rare cases. Healthy sleepers can experience up to 10 short arousals (i.e., on the order of seconds) per sleep hour. These arousals are often associated with body movements and are typically forgotten — unless they last for a few minutes, or something unusual occurs during the arousals (or is associated with them), like a specific sound or odor.
- Hauri P. Current Concepts: The Sleep Disorders, 2nd Edition, Kalamazoo, MI: Upjohn Company; 1982.