Free sleep tool
Sleep Calculator
Pick a wake-up time or a bedtime and count through full sleep cycles to a realistic window at the other end.
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 or one exact wake-up time. 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 or wake-up time, not a deadline. Missing the window is not the emergency the tool's precision might imply.
Questions
- If I go to bed at 10 PM, what time will I wake up?
- Pick "I know my bedtime," enter 10 PM, and the calculator adds your time to fall asleep plus four to six full cycles of 90 to 110 minutes each. You get a window of wake-up times, one row per cycle count. It works the same for any bedtime, including right now.
- Why does it give a range instead of one exact time?
- A full cycle runs roughly 90 to 110 minutes, and nobody knows in advance which kind of night they are about to have. One exact time would claim more precision than that. So the tool shows a range, not a promise.
- How many sleep cycles do I need a night?
- A typical night runs four to six full cycles of roughly 90 to 110 minutes each. That spans about 6 to 11 hours in total; five cycles, the middle of that range, works out to between 7.5 and about 9.2 hours. The calculator shows one row per cycle count so you can compare.
- What does "time to fall asleep" mean?
- It is the gap between getting into bed and drifting off, sometimes called sleep latency. Everyone’s gap is different, so the calculator asks for yours instead of assuming one number. Whatever you enter shifts every window by the same amount.
- Can I use it for daytime sleep, like after a night shift?
- The calculator counts the same way at any hour: bedtime, plus your time to fall asleep, plus full cycles. Enter a 7 AM bedtime and it will show wake-up windows through the afternoon. The clock position does not change the arithmetic.
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.
More sleep tools
- Caffeine Cutoff CalculatorEnter your bedtime and see when to switch to decaf, based on how long caffeine actually lingers in the body.
- Breathing PacerA visual guide for 4-7-8, box, and coherent breathing — follow the circle, slow your breath, wind down for bed.
- Tonight's SkyThe real sky tonight: the constellation the moon is in, its phase, and the constellations overhead right now.