Sleep Calculator

Answers one question fast: if I go to sleep now, when should I wake up? Sleep runs in cycles of roughly 90 minutes, alternating light and deep stages, and waking at a cycle boundary tends to feel lighter than waking mid-cycle. The tool adds about 15 minutes to fall asleep, then lists wake-up times that land on cycle boundaries — four to six cycles, or six to nine hours. It also works backwards from a fixed alarm to the time you should be in bed.

How to use

1. Choose "if I sleep now, when should I wake up?" to get suggestions from the current time immediately.
2. If your alarm is fixed, switch to the reverse mode and enter the wake-up time instead.
3. Pick the option that fits the hours you can actually afford.
4. Each option is a 90-minute boundary. Five cycles — seven and a half hours — is the usual sweet spot, but cycle length varies by person.

When you actually need this

• It is 2 a.m., you are still up, and you want the least painful wake-up time from here.
• Tomorrow's start time is fixed and you need to count backwards to a bedtime.
• Choosing a nap length — one full 90-minute cycle, or around 20 minutes to stop before deep sleep begins.
• Anchoring a consistent sleep schedule to cycle boundaries.

Where the 90-minute figure comes from — and its limits

A night of sleep repeats bundles of REM and non-REM stages, each bundle lasting roughly 80 to 110 minutes. Ninety minutes is the average, not a universal constant. Being woken from deep sleep produces the groggy weight known as sleep inertia, which is why aiming for a cycle boundary has some basis — but total sleep time matters far more than where the boundary falls. Adults are advised to get at least seven hours, and trimming sleep to hit a cycle boundary defeats the purpose. The 15-minute fall-asleep allowance is also an average; adjust your reading of the results to how quickly you actually drift off.

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FAQ

Which time zone does Sleep Calculator use?

It uses your device's local time zone and calendar settings, so the result matches the dates you see elsewhere on your computer or phone. Nothing is converted to UTC behind the scenes.

Does Sleep Calculator handle leap years correctly?

Yes. The calculation works on real calendar dates rather than a fixed 365-day year, so February 29th, month lengths and century leap-year rules are all applied correctly.

Is the start or end day included in the count?

The tool shows the elapsed days between the two dates. If you need an inclusive count — for example counting today as day one of a challenge — add one to the result.

Read the full guide: how to use Sleep Calculator step by step →