Sleep cycle calculator
Sleep runs in roughly 90-minute cycles. Waking at the end of one feels better than waking in the middle. Pick what you know and we'll do the arithmetic.
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Set the timer, then let the room go dark.
Room for Stars fades sound and screen at the wake-up time you just picked. iPhone, iPad, Apple TV.
How the calculator works
The tool adds your fall-asleep buffer to the starting time, then steps forward (or backward) in whole cycles and lists the end of each. It's arithmetic, not a diagnosis: cycle length varies between people and across the night.
Why 90 minutes
Ninety minutes is a commonly used average, not a fixed number. The Institute of Medicine's report on sleep physiology puts the first NREM-REM cycle of the night at 70 to 100 minutes, with later cycles running longer, roughly 90 to 120 minutes (Sleep Disorders and Sleep Deprivation, National Academies Press, 2006). If you want the fuller technical breakdown, freetoolbox's own sleep cycle explainer goes deeper on the stage-by-stage variance.
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