Find the best bedtime or wake-up time based on 90-minute sleep cycles, or calculate from a target sleep duration.
Sleep moves through repeating cycles of roughly 90 minutes, alternating between non-REM and REM sleep. Waking up in the middle of a cycle (during deep sleep) tends to feel groggier than waking up between cycles, so timing bedtime or wake-up around whole cycles — plus a buffer for the time it takes to actually fall asleep — can help you wake up feeling more rested.
5 to 6 complete 90-minute cycles works out to roughly 7.5–9 hours, in line with typical adult sleep recommendations. Individual needs vary, and actual cycle length isn't perfectly fixed at 90 minutes for everyone.
The Sleep Cycle Calculator gives several options timed to whole 90-minute cycles. The Sleep Length Calculator is simpler — it just adds or subtracts a specific duration you choose, without cycle alignment.
If you know it typically takes you longer (or shorter) than 15 minutes to fall asleep, adjusting this gives a more accurate bedtime recommendation.
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