FIGURE 1 | Stages of sleep in humans.

From the following article:

Sick and tired: does sleep have a vital role in the immune system?

Penelope A. Bryant, John Trinder and Nigel Curtis

Nature Reviews Immunology 4, 457-467 (June 2004)

doi:10.1038/nri1369

Sick and tired: does sleep have a vital role in the immune system?

The relationship between human sleep, level of consciousness and electroencephalogram (EEG) patterns. Stages of sleep are characterized by differences in the frequency and amplitude of EEG waves. Stage 1 comprises light sleep with low-amplitude waveforms. Stage 2 is characterized by sleep spindles (the higher-frequency waves) and K complexes (not shown). Stages 3 and 4 comprise slow-wave sleep (SWS) with high-amplitude waves and a deeper level of unconsciousness. These four stages comprise non-rapid-eye-movement (NREM) sleep. The final stage is rapid-eye-movement (REM) sleep, which is associated with theta-activity (all waves shown) and, in some individuals, with low-amplitude sawtooth waves (not shown) and a higher level of consciousness.

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