Sleep, Actually
Sleep stages and sleep cycles
A cycle runs through non-REM stages into REM and repeats several times a night, with REM periods usually lengthening toward morning.
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A sleep cycle is one pass through the stages. NHLBI describes non-REM stage 1 as the light transition into sleep, stage 2 as a longer period of lighter sleep, and stage 3 as deep sleep, followed by REM.
Cycles repeat through the night. Deep sleep tends to dominate earlier in the night and REM periods tend to get longer toward morning.
Waking briefly between cycles is normal for everyone. What sleep studies look at is the shape of the whole night, not any single awakening.
Definitions on this page
- Sleep cycle
- One pass through non-REM stages into REM, repeated across a night.
- Deep sleep (N3)
- The deepest non-REM stage, usually more prominent early in the night.
What does that mean?
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