Stages of Sleep

Sleep is not a uniform state of being. Instead, sleep is composed of several different stages that can be differentiated from one another by the patterns of brain wave activity that occur during each stage. These changes in brain wave activity can be visualized using EEG and are distinguished from one another by both the frequency and amplitude of brain waves (Figure). Sleep can be divided into two different general phases: REM sleep and non-REM (NREM) sleep. Rapid eye movement (REM) sleep is characterized by darting movements of the eyes under closed eyelids. Brain waves during REM sleep appear very similar to brain waves during wakefulness. In contrast, non-REM (NREM) sleep is subdivided into four stages distinguished from each other and from wakefulness by characteristic patterns of brain waves. The first four stages of sleep are NREM sleep, while the fifth and final stage of sleep is REM sleep. In this section, we will discuss each of these stages of sleep and their associated patterns of brain wave activity.

A photograph shows a person sleeping.  Superimposed across the top of the picture is a line representing brainwave activity across the five stages of sleep. Above the line, from left to right, it reads stage 1, stage 2, stage 3, stage 4, and stage 5. The wave amplitude is highest in late stage 2, and near the end of stage 3 through stage 4. The wavelength I longer from late stage 2 through stage 4.
Brainwave activity changes dramatically across the different stages of sleep. (credit "sleeping": modification of work by Ryan Vaarsi)
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