Free sleep tool
Sleep Calculator
Pick a wake-up time and count back through full sleep cycles to a realistic bedtime window.
Go to bed between
Each row is a window, not a target. Going to bed at the early end suits a night of long cycles; the late end assumes short ones. Nobody knows which kind of night they are about to have.
Why this gives you a range
The familiar version of this tool multiplies 90 minutes by a number of cycles and prints one exact bedtime. That precision is not real. A complete sleep cycle runs roughly 90 to 110 minutes, cycles are not identical across a night — REM periods lengthen as the night goes on — and the time it takes to fall asleep varies enormously between people and between nights.
So this calculator gives you a window per cycle count and asks you for your own falling-asleep time rather than assuming one. Treat the result as a nudge toward a sensible bedtime, not a deadline. Missing the window is not the emergency the tool's precision might imply.
Where these numbers come from
- Sleep cycle length (90 to 110 minutes), four to six cycles a night, and the lengthening of REM across the night: Physiology, Sleep Stages (StatPearls, NCBI Bookshelf).
- Time to fall asleep is deliberately not a constant here. It varies too much between people to publish as arithmetic, so it is an input you set.
This is general information, not medical advice. Sleep that stays bad for weeks is worth taking to a doctor.