Sleep stages and patterns

Sleep is a necessary part of life. It has restorative and rejuvenating properties for the brain and organ systems. The human body naturally follows a 24-hour period of wakefulness and sleepiness known as the circadian rhythm.


Sleep stages
During a good night's sleep, you will pass through the five stages. Stages one through four are considered nonrapid eye movement (NREM) sleep. Stage five is considered rapid eye movement (REM) sleep. During REM sleep, your closed eyes jerk rapidly, your limb muscles are temporarily paralyzed, and your breathing is more rapid, irregular and shallow. A complete sleep cycle lasts 90 to 110 minutes on average, although the length of time varies for each sleep stage.
Stage one is a transitional sleep stage. It's characterized by light sleep, in which you can be awakened easily, your eyes move slowly, and muscle activity slows. You may experience sudden muscle contractions often preceded by a sensation of starting to fall. You may respond to faint sounds or visual signals without awakening. It accounts for 5 percent of total sleep time.

During stage two, eye movements stop and brain waves become slower. This stage comprises about 50 percent of total sleep time. The amount of time in stage two increases with age. Older adults spend a majority of their total sleep time in stages one and two.

Stages three and four are considered deep-sleep stages. During stage three, extremely slow brain waves called delta waves begin to appear. Stage four includes almost exclusively delta waves. There's no eye movement or muscle activity. If awakened during stage four, you won't adjust immediately and may feel groggy and disoriented for several minutes. Some children experience bedwetting, night terrors or sleepwalking during these stages of deep sleep. Stage three accounts for 3 to 8 percent of total sleep time, and stage four accounts for 10 to 15 percent.
About 20 to 25 percent of adult sleep time is spent in REM sleep, which usually begins 70 to 90 minutes after initially falling asleep. Infants spend about half of their sleep time in REM sleep.

Awakening during REM sleep is often associated with a vivid recall of dreams. Dream recall increases if you're awakened during later REM sleep episodes of the sleep cycle. If you're awakened during NREM (stages one through four), dream recall can occur, but it's less vivid and lacks the detail of REM dream recall.

The first sleep cycles of the night contain relatively short REM periods and long periods of deep sleep (stages three and four). As the sleep progresses, REM sleep periods increase in length while length of deep sleep decreases. By the end of a night's sleep, nearly all your sleep is in stage one and REM sleep.

Most people need about eight hours of sleep a night, but individual requirements vary. And, of course, adequate sleep is partly subjective. Some people who feel well rested sleep less than those who complain of sleep deprivation. In some cases, insomniacs may actually underestimate the amount of sleep they truly get each night. Often, those who complain about their sleep quality and amount are actually getting more sleep than those who do not complain.