Sleep Calculator
Find the best bedtime or wake-up time based on 90-minute sleep cycles.
Enter your values to calculate your result.
Formula
Candidate Time = Target Time ± (Fall-Asleep Buffer + Whole Sleep Cycles × 90 minutes)
Sleep progresses through cycles of roughly 90 minutes each. Waking up at the end of a complete cycle (rather than in the middle of one) tends to feel more refreshing. This calculator offers a recommended option of 5 full cycles (7.5 hours), plus a longer (6 cycles, 9 hours) and shorter (4 cycles, 6 hours) alternative, each adjusted by the time it typically takes to fall asleep.
Example
If you need to wake up at 7:00 AM and typically take 15 minutes to fall asleep, the recommended bedtime (5 cycles) is about 11:15 PM the night before.
Frequently Asked Questions
Why 90-minute cycles?
A full sleep cycle — moving through light sleep, deep sleep, and REM sleep — takes roughly 90 minutes on average, though this varies somewhat by individual.
Is more sleep always better?
Most adults need about 7–9 hours per night. The 'longer' and 'shorter' options are provided as alternatives within a reasonable range, not as a recommendation to sleep as little as possible.