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Sleep calculator. Wake between cycles, not mid-dream.

Read the written guide A free online tool by Digital Heroes

To wake up at 7:00 AM, head to bed at one of these times. Each option is a whole number of sleep cycles, the highlighted ones are the sweet spot.

Runs entirely in your browser, nothing is uploaded. Only your mode and fall-asleep preference are saved locally.

Why 90 minutes?

Sleep moves through repeating cycles of roughly 90 minutes, each running from light sleep down into deep sleep and REM, then back toward light sleep. Waking at the end of a cycle, when you're already near the surface, tends to feel far easier than being jolted awake from deep sleep, even if you've slept slightly less overall.

Most adults do best on 5-6 cycles (about 7.5-9 hours). We also add ~15 minutes to fall asleep, since you don't drift off the instant your head hits the pillow. These are friendly estimates, cycle length varies from person to person, so treat the highlighted picks as a starting point and adjust to what leaves you feeling rested.

§ · written guide

How to use the Sleep Calculator

This calculator suggests bedtimes or alarm times that line up with whole sleep cycles. It counts in 90 minute cycles and adds the time you usually take to fall asleep, so you wake near the end of a cycle rather than in the middle of one. Use it to plan tonight's bedtime around a fixed alarm, or to set an alarm when you are about to sleep.

Step by step

  1. Pick a mode

    Choose I want to wake up at if your alarm time is fixed, or I'm going to sleep if your bedtime is. The first mode is selected on your first visit.

  2. Set your wake-up time

    In the first mode, enter your Wake-up time. It starts at 7:00 AM. The four bedtime cards update when you finish changing the time.

  3. Or set when you go to sleep

    In the second mode, leave Going to sleep on Right now to count from the current time, or choose At a specific time and enter a Bedtime, which starts at 11:00 PM. In Right now mode the times refresh every 30 seconds.

  4. Adjust the time to fall asleep

    Set Time to fall asleep to how long you usually lie awake: Instantly, or about 5, 10, 15, 20 or 30 minutes. The default is 15. Every suggestion shifts by this amount.

  5. Pick a card

    Each card shows a time, the number of cycles and the hours of sleep. Cards marked Recommended give 5 or 6 cycles. Set your alarm or head to bed at the time on the card you choose.

Reading the result

  • Cards run from 6 cycles down to 3. A cycle is 90 minutes, so 6 cycles is 9 hrs of sleep, 5 is 7 hrs 30 min, 4 is 6 hrs and 3 is 4 hrs 30 min.
  • With the defaults, to wake at 7:00 AM the Go to bed cards read 9:45 PM, 11:15 PM, 12:45 AM and 2:15 AM. Each one is the wake time minus the sleep and the 15 minutes to fall asleep.
  • In the second mode the Wake up cards add the same amounts forward. Going to bed at 11:00 PM gives 8:15 AM, 6:45 AM, 5:15 AM and 3:45 AM.
  • The bar on each card fills one segment per cycle out of six. The 3 cycle card has an amber label to mark a very short night.

Tips and common mistakes

  • Ninety minutes is an average. If you wake groggy at a card's time, try the next card or change the time to fall asleep by 10 minutes and see how you feel over a few nights.
  • Be honest about the time to fall asleep. If you read or scroll in bed, count that too.
  • Times are shown in your device's clock format and time zone, so a 24 hour device shows 21:45 instead of 9:45 PM.
  • This is a planning aid, not medical advice. If you feel tired after full nights for weeks, talk to a doctor.

Privacy and limits

All the calculations run in your browser and nothing is sent anywhere. Only your chosen mode and your time to fall asleep are saved in this browser's local storage, so they are remembered next time and stay in step across open tabs. The wake-up and bedtime you type are not saved.

Back to the tool

§ 02 · what you can do

Better mornings, by arithmetic.

  • Find the ideal bedtime by entering your target wake-up time, four cycle-aligned options appear at once.
  • Find the best wake-up time by telling the tool when you're going to sleep, including a “right now” mode.
  • See cycles and total sleep for every option, from 3 cycles (minimum) to 6 cycles (optimal), each with a visual cycle bar.
  • Adjust your fall-asleep delay from instantly to ~30 minutes so the math reflects how long you actually take to drift off.
  • Spot the sweet spot at a glance: the 5- and 6-cycle options carry a Recommended tag; very short nights are flagged in amber.
  • Stay current automatically: in sleep-now mode the times refresh every 30 seconds, and your preferences persist locally and sync across open tabs.

If you are planning a wellness app with features like this one, see the health apps we build.

§ 04 · faq

Frequently asked questions.

Is the Sleep Calculator free?
Yes, completely free for everyone. No sign-up, no account and no usage limits.
Is it private? Is anything uploaded?
No upload, ever. All the math runs in your browser on your own device. The only thing saved is your mode and fall-asleep preference, stored locally so the tool remembers them next time.
Why does it use 90-minute cycles?
Sleep runs in repeating cycles of roughly 90 minutes, moving from light sleep into deep sleep and REM and back. Waking at the end of a cycle, when you're already near the surface, usually feels much easier than being pulled out of deep sleep mid-cycle.
How many sleep cycles do I need?
Most adults do best on 5-6 cycles per night, about 7.5 to 9 hours of sleep. The tool shows options from 3 to 6 cycles and highlights the 5- and 6-cycle times as Recommended. Cycle length varies from person to person, so treat them as a starting point.
What does “time to fall asleep” change?
It's the buffer between getting into bed and actually being asleep. The default is ~15 minutes, adjustable from instantly to ~30 minutes; every bedtime and alarm suggestion shifts by that amount so the cycles line up with real sleep, not lights-out.
Does it work offline?
Yes, once the page has loaded, all calculations run locally in your browser, so switching modes, changing times and reading results keeps working without a connection.

Published · Last updated .

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