Sleep
The measurement of eye movement during sleep is used to divide sleep into the two broad types of Rapid Eye Movement (REM) and Non-Rapid Eye Movement (NREM) sleep. Each type has a distinct set of associated physiological, neurological and psychological features.
Sleep proceeds in cycles of REM and the four stages of NREM. In humans this cycle is on average 90 to 110 minutes, with a greater amount of stages 3 and 4 early in the night and more REM later in the night.
Hours of sleep needed by age group
Children need a greater amount of sleep per day than adults to develop and function properly: up to 18 hours for newborn babies, with a declining rate as a child ages. A newborn baby spends almost half of its sleep time in REM-sleep. By the age of five or so, only a bit over two hours are spent in REM.
| Age | Average amount of sleep per day |
| Newborn | Up to 18 hours |
| 1-12 months | 14-18 hours |
| 1-3 years | 12-15 hours |
| 3-5 years | 11-13 hours |
| 5-12 years | 9-11 hours |
| Adolescents | 9-10 hours |
| Adults and Elderly | 7-8 (+) hours |
| Pregnant women | 8 (+) hours |
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