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Sleep stages decoded in 5 fast facts

N1 is the lightest non-REM stage, a brief transition from wakefulness, often just a few minutes.

— [1][2][3][4]

N2 is the stage with sleep spindles and K-complexes, often 10 to 25 minutes in the first cycle.

— [5][6][7][8][9]

N3 is the deepest non-REM sleep, or slow-wave sleep, and is most concentrated early at night.

— [10][11][12][13]

REM brings vivid dreaming, rapid eye movements, and temporary muscle atonia, usually starting about 90 minutes after sleep begins.

— [14][15][16][17]

Micro-arousals are brief wake intrusions during sleep, commonly lasting about 3 to 15 seconds.

— [18][19][20][21]

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