Understanding sleep cycles
Sleep isn't a uniform block: it's a succession of ~90-minute cycles, each moving through several phases.
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The phases in detail
Each cycle starts with light sleep, deepens into restorative deep sleep, then ends with REM sleep. This pattern repeats 4 to 6 times a night.
The impact on waking up
Waking up during deep sleep causes marked sleep inertia. This is the principle behind our sleep cycle calculator.
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