The Different Stages of Sleep
In 1952, sleep-researcher Nathaniel Kleitman discovered that although sleepers tended to have slow, rolling eye movements beneath their lids as they fell asleep, during some portions of their sleep their eyes darted rapidly in a highly coordinated way, moving more quickly and sharply than they could while they were awake. He dubbed the phenomenon rapid eye movement (REM), a phase of sleep that was later related to dreaming.
Later researchers using electroencephalogram (EEG) that measures the electrical activity in the brain discovered that the REM stage of sleep is different from non-REM (NREM) sleep.
Sleep occurs in a series of cycles, each lasting between sixty and ninety minutes. A normal sleep pattern involves four to seven such cycles during the course of the night. On average, people have five or six sleep cycles during a normal nighttime sleep session.
Each cycle has two main parts.
During the first part, our level of consciousness falls while the level of unconsciousness rises. This part of the cycle involves changes in heart rate and breathing, and an overall slowing of brain activity. We do not dream during this phase.
In the second part of the cycle, however, we do dream. The characteristic sign of this phase of sleep is rapid eye movement, or REM.
Generally, each ninety-minute sleep cycle contains a non- REM period (or slow-wave sleep) and the REM period. On average, each of these two main periods occupies about 50 percent of the cycle's elapsed time, or about forty-five minutes. However, the balance between the two periods shifts during the course of the night. During the first ninety-minute cycle, the REM phase might last only a few minutes. In the final cycle of the night, REM sleep occupies most of the time, perhaps an hour or more.
Non-REM sleep actually consists of four distinct substages, labeled 1 through 4. The stages are defined according to the types and intensity of brain wave activity as measured by EEG tracings, and also by the ease with which the sleeping person can be aroused.
The initial stage of sleep is known as the threshold. The process of falling asleep in actuality occurs in seconds. When you switch over into sleep, your awareness of the outside world vanishes in seconds.
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