Sleep Calculator
Tell us when you need to wake up and we suggest bedtimes. Or tell us when you are going to bed and we suggest wake-up times.
This calculator gives general estimates for information only. It is not medical advice and does not replace a consultation with a doctor or other qualified health professional.
Be the first to rate this tool
The formula
Wake time = bedtime + 15 minutes to fall asleep + (number of cycles x 90 minutes).
Why 90 minutes
Sleep moves through light, deep and REM stages in cycles that last about 90 minutes. Waking up at the end of a cycle, when sleep is lightest, tends to feel easier than being pulled out of deep sleep.
How many cycles do I need?
Most adults need 7 to 9 hours, which is five or six cycles. Four cycles (6 hours) can work occasionally but is not enough for most people over time. We list three to six cycles so you can pick the one that fits your night.
Example
To wake at 7:00 with six cycles, count back 9 hours plus 15 minutes to fall asleep, and you get 21:45. For five cycles, go to bed at 23:15.
Small habits that help
- Keep a steady wake-up time, even on weekends.
- Cut caffeine eight hours before bed.
- Keep the room dark, cool and quiet.
The 90-minute rule is an average. If you struggle with sleep for weeks, speak to a doctor.
Examples to check
Type these into the calculator above to see how it behaves. Every result below was produced by the same calculator code that runs on this page.
| What you enter | What you get |
|---|---|
| Wake up at a set time (find bedtimes); Time: 07:00 | 6 cycles (9 h sleep): 21:45; 5 cycles (7.5 h sleep): 23:15 |
| Wake up at a set time (find bedtimes); Time: 06:00 | 6 cycles (9 h sleep): 20:45; 5 cycles (7.5 h sleep): 22:15 |
| Go to bed now or at a set time (find wake times); Time: 23:00 | 6 cycles (9 h sleep): 08:15; 5 cycles (7.5 h sleep): 06:45 |
Sleep Calculator: frequently asked questions
Is 90 minutes exact?
It is an average. Cycles vary between 80 and 110 minutes and from night to night, so treat the times as a good starting point.
What if I take longer than 15 minutes to fall asleep?
Add that extra time to your bedtime.
Can I rely on this result as medical advice?
No. It is a general estimate for information only. It cannot replace a check-up, and your doctor can interpret the numbers for your own situation.
Is what I enter in this calculator saved or sent anywhere?
No. The calculation runs in your browser, so what you enter is not sent to our servers. We only count, anonymously, whether a tool ran successfully.
Is this calculator free to use?
Yes. It is free, needs no sign-up and has no usage limit.
Last reviewed: October 3, 2026