Sleep calculator

Find a bedtime or wake-up time for tonight using a clear sleep-cycle model.

Device timezone: detecting…. Planning estimate only. Not medical advice.

No account and no health data uploaded
Adjustable latency and cycle length
Works instantly in your browser

Simple method

Choose a time. Compare the trade-offs.

Sandy adds a realistic time to fall asleep, then compares complete sleep cycles. You can adjust both assumptions and see the limits of the model.

Read the full calculation method →

90 min: Default cycle estimate. Adjust it from 80 to 120 minutes.
15 min: Default time to fall asleep. Adjust it from 5 to 45 minutes.
4 options: Compare nearby times instead of treating one result as exact.

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Popular sleep guides

Understand the model, its limits and the practical questions behind the numbers.

Sleep scienceHow sleep cycles workWhat a 90-minute cycle means, why it varies, and how to use it as a planning model.ReferenceSleep duration by ageA clear reference table for children, teenagers and adults, with source links.Practical guideHow to take a better napShort naps, full-cycle naps and the trade-offs that matter in the afternoon.MethodologyHow this calculator worksThe exact formula, the assumptions behind it, and what it cannot tell you.

Questions before you plan.

Clear answers about the model, privacy and the limits of a scheduling tool.

Is a 90-minute sleep cycle accurate for everyone?

No. Sleep cycles usually last around 70 to 120 minutes and vary by person and by night. The 90-minute value is a practical planning estimate, not a measurement of your sleep.

Why does the calculator add 15 minutes?

It assumes a short period to fall asleep. You can change that assumption under “Adjust sleep assumptions” if your experience is different.

Does this website upload my sleep times?

No. The calculation happens in your browser. The tool does not ask for an account and does not send your selected times to a server.

Is this medical advice?

No. It is a general planning tool. Persistent insomnia, extreme daytime sleepiness, snoring with breathing pauses, or other health concerns should be discussed with a qualified professional.

Why show several options instead of one exact time?

Sleep timing is not exact. Giving a small set of options makes the uncertainty visible and helps you choose a bedtime that fits your real schedule.