Why 90-minute cycles?

A full sleep cycle, moving through light, deep, and REM sleep, averages about 90 minutes. Waking at the end of one, rather than the middle, is why you sometimes feel refreshed after less sleep and groggy after more.

How many cycles do I need?

Most adults do well with 5 to 6 cycles, which is about 7.5 to 9 hours. This tool shows bedtimes for both so you can choose what fits your night.

Verified formulaLast reviewed July 2026

Sleep Calculator

Find the best bedtimes to wake up refreshed at the end of a sleep cycle.

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A mathematical estimate based on validated equations, for education only. It is not a diagnostic tool.

The science

Sleep runs in cycles of roughly 90 minutes, and waking at the end of a cycle tends to feel far better than waking in the middle of one. Tell this calculator when you need to wake up, and it suggests bedtimes that let you complete whole cycles, with a little time to fall asleep built in.

The formula

Bedtime = wake time minus whole 90-minute cycles, plus 15 minutes to fall asleep

Sources

1
National Sleep Foundation sleep-cycle guidance
2
General sleep-cycle physiology (about 90 minutes per cycle)

Common questions

This tool is for education only and is not a diagnostic device. Consult a licensed clinician before making health decisions.