Sleep Calculator
Find the best time to go to bed or wake up from 90-minute sleep cycles and the time it takes you to fall asleep. Free sleep cycle calculator, no sign-up.
Estimate based on published formulas, not medical advice. Talk to a health professional about your situation.
About 90 minutes per cycle and 15 minutes to fall asleep are typical averages; cycles vary from about 70 to 120 minutes.
Go to bed at one of these times
How to use the sleep calculator
- Under What do you know?, choose I want to wake up at… to get bedtimes, or I’m going to bed at… to get wake-up times.
- Enter the Time and click Calculate. The results also update as you type.
- Going to bed right away? Click I’m going to bed now: the tool uses your device’s current time and shows when to set your alarm.
- Adjust Sleep cycle length (minutes) and Time to fall asleep (minutes) if you know your own habits. The defaults are 90 and 15 minutes.
The tool lists four options: 3, 4, 5 and 6 complete cycles. The 5- and 6-cycle options are marked recommended, because with 90-minute cycles they give 7.5 and 9 hours of sleep, which matches what most adults need.
How sleep cycles work
During the night, sleep is not uniform. You move through repeating cycles of light sleep (stages N1 and N2), deep slow-wave sleep (N3) and REM sleep, the stage most associated with vivid dreaming. A full cycle lasts about 90 minutes on average, and a typical night contains four to six of them. Deep sleep dominates the first cycles, while REM periods get longer towards the morning.
Waking up at the end of a cycle, when sleep is lighter, often feels easier than being pulled out of deep sleep, which can leave you groggy for a while (this grogginess is called sleep inertia). That is the idea behind this calculator: it counts back from your wake-up time, or forward from your bedtime, in whole cycles.
The calculation is simple:
- Bedtime = wake-up time − (number of cycles × cycle length) − time to fall asleep
- Wake-up time = bedtime + time to fall asleep + (number of cycles × cycle length)
For example, to wake up at 7:00 with 90-minute cycles and 15 minutes to fall asleep, 6 cycles (9 hours) means going to bed at 21:45, and 5 cycles (7 hours 30 minutes) means 23:15.
Limits: cycles are not a clock
The 90-minute figure is an average. Real cycles vary from about 70 to 120 minutes, from one person to another and from one cycle to the next in the same night. Stress, alcohol, caffeine, noise and an irregular schedule change them too. Treat the results as a starting point, not as a precise schedule: if you often wake up just before your alarm, or feel groggy at a given time, try a different cycle length or falling-asleep time. This tool gives estimates only and is not medical advice.
How much sleep do you need?
The total amount of sleep matters more than the exact minute you wake up. The National Sleep Foundation’s recommendations (Hirshkowitz et al., Sleep Health, 2015) and the Canadian 24-Hour Movement Guidelines give these ranges:
| Age | Recommended sleep per 24 hours |
|---|---|
| Newborns (0–3 months) | 14–17 hours |
| Infants (4–11 months) | 12–15 hours |
| Toddlers (1–2 years) | 11–14 hours |
| Preschoolers (3–5 years) | 10–13 hours |
| Children (6–13 years) | 9–11 hours |
| Teenagers (14–17 years) | 8–10 hours |
| Adults (18–64 years) | 7–9 hours |
| Older adults (65 years and over) | 7–8 hours |
Keeping regular bed and wake times, even on weekends, helps as much as choosing the right cycle.
Frequently asked questions
What time should I go to bed to wake up at 6:00?
With 90-minute cycles and 15 minutes to fall asleep: 20:45 for 6 cycles (9 hours) or 22:15 for 5 cycles (7.5 hours). Enter 6:00 in the tool to see all the options.
Why does the tool add 15 minutes?
Most healthy adults take about 10 to 20 minutes to fall asleep. The tool adds this delay so that the cycles start when you actually fall asleep, not when you get into bed. You can change it under Time to fall asleep (minutes).
Is a 90-minute cycle the same for everyone?
No. It is an average: cycles usually last between 70 and 120 minutes and change during the night. If you know your own rhythm, for instance from experience, adjust Sleep cycle length (minutes).
Is 3 or 4 cycles enough?
Occasionally, it may help you wake up less groggy after a short night, but 4.5 or 6 hours is below the 7 to 9 hours recommended for adults. Regular short nights are linked to health problems, so aim for 5 or 6 cycles.
Does the calculator track my sleep?
No. It only does arithmetic on the times you enter, in your browser. Nothing is stored or sent. If you have ongoing trouble sleeping, talk to a health professional.
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