The Sleep Cycle

The 5 Stages of Sleep

You usually pass through five phases of sleep while sleeping: stages 1, 2, 3, 4, and REM (rapid eye movement) sleep.

These stages progress in a cycle from stage 1 to REM sleep and then repeat in order. Almost 50% of your total sleep time is in stage 2 sleep, about 20% in REM sleep, and the remaining 30% in the other stages.

By contrast, infants spend about half of their sleep time in REM sleep.

Each of the stages of sleep have different characteristics.
 
Stage 1: Light Sleep. Typically you drift in and out of sleep and can be awakened easily. Your eyes move slowly and muscle activity slows. People awakened from stage 1 sleep often remember fragmented visual images. Many also experience sudden muscle contractions called hypnic myoclonia, often preceded by a sensation of starting to fall. These sudden movements are similar to the "jump" we make when startled.

Stage 2:  Eye movements stop and brain waves become slower, with occasional bursts of rapid brain waves called sleep spindles.

Stage 3: Delta waves (extremely slow brain waves) appear, interspersed with smaller, faster waves.

Stage 4: Delta waves are produced by your brain almost exclusively. You are difficult to wake up during stages 3 and 4, which together are called deep sleep. You exhibit no eye movement or muscle activity. If you are awakened during deep sleep, you do not adjust quickly and may feel groggy and disoriented for several minutes after waking. Some children experience bedwetting, night terrors, or sleepwalking during deep sleep.

REM Sleep: Breathing increases while becoming irregular and shallow. Eyes move rapidly in various directions, and limb muscles become temporarily paralyzed. Heart rate increases, blood pressure rises, and males develop penile erections. When people awaken during REM sleep, they often describe bizarre and illogical tales we call "dreams."

The first REM sleep period usually occurs about 70 to 90 minutes after you fall asleep. A complete sleep cycle takes 90 to 110 minutes on average.

The first sleep cycle each night contains relatively short REM periods and long periods of deep sleep. As the night progresses, REM sleep periods increase in length while deep sleep decreases. By morning, people spend nearly all their sleep time in stages 1, 2, and REM.