These stages of sleep occur in cycles that last approximately 90 to 110 minutes, and which repeat four to five times a night.Once asleep, we experience a sleep cycle that consists of several stages, each with its own distinctive pattern Of brain waves.The sleep consists of five stages.
Stages Of Sleep Cycle
Stage I involves falling asleep, the stage that usually occurs a few minutes after we climb into bed. As the rhythmic pattern of alpha waves that characterizes the relaxed waking state begins to disappear, the brain waves become irregular. The heart rate slows and the muscles relax. People in Stage I are easy to waken and may not even realize that they have fallen asleep.
In Stage 2, the brain waves show bursts of activity called “spindles” because their tracings on an EEG chart resemble thread wrapped around an old-fashioned spindle. Stage 2 is a deeper stage of sleep than Stage l.
In Stage 3, even deeper sleep, the spindle-shaped brain waves are replaced by long, slow delta waves. The sleeper is difficult to waken and unresponsive to external stimuli. Heart rate, blood pressure, and temperature continue to drop.
Stage 4, the stage of deepest sleep, is also called “delta sleep” because it is dominated by delta waves as slow as one per second. In young adults, delta sleep recurs in 15- to 30-minute segments, interspersed with lighter stages, during the first half of the night. With age, one’s quota of delta sleep diminishes. If you miss a night of sleep—for example, by staying up to write a paper much of the following night will be devoted to delta sleep.
REM (Rapid Eye Movement) Sleep
In the course of a sleep cycle, sleepers proceed predictably through the stages of sleep from Stage I to Stage 4 and then retrace their journey back to Stage l. Upon arriving back at Stage l, however, sleepers do not awaken. Instead, they enter what is called REM (rapid eye movement) sleep, the stage during which vivid dreams occur.
The first period of REM sleep usually begins about 90 minutes after we fall asleep and lasts 5 to 10 minutes. Later in the night, REM periods last as long as 25 minutes. REM sleep is readily identifiable by looking closely at a sleeper’s eyelids: the eyes are darting and rolling around under the closed eyelids, possibly tracking the events occurring in a dream. Because such eye movements do not occur in Stages 2 through 4, those stages are referred to as non-REM (or NREM) sleep.
With each cycle through the stages of sleep, the REM period typically be- comes longer and the slow-wave stages shorter. During the latter part of the night, the sleeper usually reverts to REM sleep after Stage 2, without returning to Stages 3 and 4. These cycles last about 90 minutes. people who are awakened during REM sleep nearly always report that they have been dreaming. By contrast, people awakened during non-REM sleep only occasionally report dreams, and those they describe are typically closer to random thoughts than to vivid dreams. Apparently dreams during non-REM sleep are less vivid or more easily forgotten.