Sleep Calculator
Bedtimes and wake times in whole sleep cycles.
Sleep Calculator works backwards from your alarm, or forwards from bedtime, in whole 90-minute cycles plus the time you take to fall asleep. Each option shows its cycle count and total sleep, and is marked against the recommended range for your age. Everything is worked out in your browser and nothing is uploaded.
Loading Sleep Calculator…
Runs entirely on your device. No file or text you enter here is uploaded. How to verify it.
This is a calculation tool, not medical advice. The result is an estimate from a published formula and cannot account for your individual circumstances. Talk to a healthcare professional about anything that matters.
How to use Sleep Calculator
- Choose whether you know your waking time or your bedtime.
- Set that time, your age band, and how long you take to fall asleep.
- Pick an option that lands inside the recommended range.
Questions about Sleep Calculator
Are sleep cycles really 90 minutes?
Ninety is an average, not a constant. Measured cycles run from roughly 70 to 120 minutes, they differ from person to person, and they lengthen across the night as deep sleep gives way to REM. The cycle length here is adjustable precisely because the textbook figure fits few people exactly.
Why does the tool add time to fall asleep?
Because a bedtime is not the moment you fall asleep. Fifteen to twenty minutes is typical for an adult, and the tool adds it to time in bed rather than to the sleep total, so a row promising seven and a half hours of sleep asks for seven and three quarter hours in bed.
Should I cut sleep short to avoid waking mid-cycle?
Almost never. Waking from deep sleep does leave you groggy for anything up to half an hour, which is called sleep inertia and is quite real, but the grogginess passes and the lost sleep does not. Six hours timed to a boundary is worse than seven and a half that ends mid-cycle.
Does this work for children and babies?
The recommended ranges cover every age band from newborn upwards, with naps included for the younger ones. The cycle arithmetic fits far less well there: an infant’s cycle is nearer 50 minutes than 90, and sleep is scattered across the day rather than gathered into one block.
Is anything I enter here saved?
No. The time, the age band and the cycle settings fill in the arithmetic and are gone when the tab closes. There is no account, no upload and nothing written to storage, so the tool keeps no record of when you sleep.
Is Sleep Calculator really free, with no limits?
Yes. There is no account, no daily cap, no file-size tier and no watermark, because there is no server cost to recover. The site is funded by clearly disclosed partner recommendations on the Partners page, never by metering the tools.
Are my files or text uploaded when I use Sleep Calculator?
No. Everything is processed by JavaScript running in your browser, on your device. You can verify it: open your browser's developer tools, switch to the Network tab, and use the tool. You will see no request carrying your data — because there is nowhere for it to go.