Sleep calculator
Tell it when you need to be up. It suggests bedtimes that avoid waking mid-cycle.
- Runs in your browser — nothing you enter is uploaded
- No sign-up, no quotas, free to use as often as you like
How it works
Waking in the middle of a deep stage of sleep feels far worse than waking after the same amount of sleep at the end of a cycle. Timing sleep in whole cycles is a rough way to avoid that, and rough is genuinely better than nothing here.
The idea
Sleep runs in cycles of roughly 90 minutes, moving from light into deep sleep and back out again. Waking near the end of a cycle, when sleep is lightest, tends to feel easier than waking 40 minutes into one — even if the second option gave you more sleep overall. The suggested times are whole numbers of cycles, plus an allowance for how long you take to fall asleep.
Three ways to ask
Work backwards from the time you must be up, forwards from a bedtime you have in mind, or forwards from right now — which is the version you want at midnight when the real question is whether to go to bed immediately.
Where the honesty is needed
Ninety minutes is an average, not a constant. Real cycles range from about 70 to 120 minutes, vary between people, and change across the night and with age, alcohol, stress and sleep debt. Treat these as a starting point rather than an instruction, and adjust by how you actually feel.
Falling asleep
The default allowance is fifteen minutes. If you know you take longer, change it — it shifts every suggestion by the same amount.
Sleep calculator — questions
Is the 90-minute cycle exact?
No. It is an average; real cycles vary between roughly 70 and 120 minutes and change through the night. The tool says so rather than implying precision it does not have.
How many cycles should I aim for?
Five or six for most adults, which is seven and a half to nine hours. Four can work occasionally; three is a short night.
Does this work for naps?
Partly. A 20-minute nap before deep sleep begins, or a full 90-minute cycle, both tend to feel better than waking at 45 minutes.
Is this medical advice?
No. It is arithmetic on an average cycle length. Persistent sleep problems are worth raising with a doctor.
Is anything sent anywhere?
No. It runs in your browser.