About this sleep calculator
Sleep tends to run in cycles of roughly 90 minutes, moving from light sleep through deeper stages and back again. Waking at the end of a cycle, rather than in the middle of one, is the idea behind this calculator: you pick a wake-up time and it counts backwards in whole cycles, or you pick a bedtime and it counts forwards. The calculator works in complete cycles only — 3, 4, 5 or 6 of them, which is 4.5, 6, 7.5 or 9 hours of sleep — and it adds the time you normally take to fall asleep, because the cycles only start once you are actually asleep. Five or six cycles suit most adults, so those two results are marked as recommended.
For a 6:00 AM wake-up with 15 minutes to fall asleep, the bedtimes are 8:45 PM (6 cycles, 9 hours), 10:15 PM (5 cycles, 7.5 hours) and 11:45 PM (4 cycles, 6 hours). Change the wake-up time or the fall-asleep time and every result updates to match.
How to use it
- Choose what you want to plan. A fixed wake-up time, a fixed bedtime, or sleeping right now.
- Enter your time. Type your wake-up time or bedtime, or leave “sleep now” to use your device clock.
- Set your fall-asleep time. The default is 15 minutes; adjust it between 5 and 30 minutes to match how long you usually take to drift off.
- Press Calculate. You get four times, one per cycle count, with the 5- and 6-cycle options marked as recommended for most adults.
- For a daytime nap, use Nap mode: enter when your nap starts and choose the 20-minute or 90-minute alarm time.
Important disclaimer
This calculator is a planning aid only, not medical advice. Sleep cycles average around 90 minutes, but real cycles vary from roughly 70 to 120 minutes, and they change through the night and from person to person. Nothing on this page can diagnose a sleep problem, and no result here guarantees how you will feel when you wake up. If you regularly struggle to fall asleep, stay asleep, or feel tired during the day, speak to a doctor or a qualified sleep professional.
Frequently asked questions
What time should I go to bed if I wake up at 6:00 AM?
Counting back in 90-minute cycles with 15 minutes to fall asleep, the bedtimes are 8:45 PM (6 cycles, 9 hours), 10:15 PM (5 cycles, 7.5 hours), 11:45 PM (4 cycles, 6 hours) and 1:15 AM (3 cycles, 4.5 hours). Most adults should aim for the 5- or 6-cycle options.
How many sleep cycles do most adults need?
Five or six full cycles a night — 7.5 or 9 hours of sleep plus the time it takes to fall asleep — suits most adults. Some people genuinely need a little more or a little less, and age, health and daily demands all shift the number, so use these figures as a starting point and pay attention to how you actually feel.
Is a 20-minute nap or a 90-minute nap better?
It depends on what you need the nap for. A 20-minute nap is a short rest that usually ends before deep sleep begins. A 90-minute nap covers roughly one full cycle, so you are more likely to wake at a lighter stage. Naps that land in between — around 30 to 60 minutes — are the ones most likely to leave you groggy, which is why this calculator offers exactly the 20-minute and 90-minute options.
Why does the calculator ask how long it takes me to fall asleep?
Because sleep cycles are counted from the moment you actually fall asleep, not from when you get into bed. If it takes you 15 minutes to drift off, a 7.5-hour plan needs you in bed 7 hours and 45 minutes before your alarm. The default is 15 minutes, and you can change it to anything from 5 to 30 minutes.
Related tools
What Time Should I Go to Bed? — the full guide behind this calculator. Timestamp Converter — convert dates and times between formats. Date Difference Calculator — days between any two dates. BMI Calculator — check your body mass index.