Sleep calculator — best bedtimes and wake-up times
Find bedtimes and wake-up times aligned to 90-minute sleep cycles, including the time it takes you to fall asleep. Works in both directions.
- Runs in your browser
- Nothing you enter is sent
- Free, no account
Reading your device clock…
How to use it
- If you have a fixed alarm, use I must wake up at… and pick one of the suggested bedtimes.
- If your bedtime is fixed, use I go to bed at… and set your alarm to one of the suggested wake times.
- Going to sleep right now? The third tab counts from the current time on your device.
- Adjust the two settings if you know your own numbers — how long you take to fall asleep, and how long your cycles run.
What a sleep cycle is
Sleep is not uniform. Across the night you move repeatedly through light sleep (stages N1 and N2), deep slow-wave sleep (N3) and REM sleep, then briefly surface before starting again. One full pass averages around 90 minutes in adults, which is the figure this calculator uses by default.
Waking at the end of a cycle, when you are already in light sleep, generally feels easy. Waking from deep sleep produces sleep inertia — that heavy, disoriented grogginess that can last twenty to thirty minutes regardless of how much sleep you got in total. Timing your alarm to land between cycles is the whole idea behind the times suggested above.
How accurate is 90 minutes?
It is an average, and the honest answer is that individual variation is large. Real cycles range from about 70 to 120 minutes, and they change through the night: early cycles are richer in deep sleep and tend to be shorter, later ones contain more REM and stretch out. Age, alcohol, caffeine, stress, screens and sleep debt all shift the pattern.
Treat the suggestions as sensible starting points rather than precision instruments. If you consistently wake groggy at the 90-minute suggestions, try setting the cycle length to 85 or 100 minutes and compare — that setting exists precisely because one number cannot fit everyone.
The fall-asleep allowance
Sleep latency — the time between lights out and actually sleeping — averages 10 to 20 minutes in healthy adults, so the default here is 15. Getting this figure roughly right matters more than people expect, because an error carries through to every cycle boundary.
Two useful signals: consistently falling asleep in under five minutes usually indicates significant sleep deprivation rather than good sleep, and regularly taking more than 30 minutes is one of the diagnostic markers for insomnia.
How much sleep is enough?
| Age group | Recommended sleep | Roughly, in 90-minute cycles |
|---|---|---|
| Newborn (0–3 months) | 14–17 hours | cycles are 50–60 min at this age |
| Infant (4–11 months) | 12–15 hours | — |
| Toddler (1–2 years) | 11–14 hours | — |
| Preschool (3–5 years) | 10–13 hours | — |
| School age (6–13 years) | 9–11 hours | 6–7 cycles |
| Teenager (14–17 years) | 8–10 hours | 5.5–6.5 cycles |
| Adult (18–64 years) | 7–9 hours | 5–6 cycles |
| Older adult (65+) | 7–8 hours | 5–5.5 cycles |
Figures follow the widely used National Sleep Foundation consensus ranges. Five or six full cycles — 7.5 to 9 hours — is the target for most adults, which is why those options are marked as recommended above.
Getting more out of the suggested times
- Keep the schedule constant. A regular wake time, including at weekends, does more for sleep quality than any single night's timing.
- Do not chase a cycle boundary at the cost of an hour. Six hours of sleep beats staying up another 90 minutes to hit a "perfect" wake time.
- Naps work in cycles too. A 20-minute nap stays in light sleep and refreshes; a 90-minute nap completes a cycle. A 45-minute nap tends to end in deep sleep and feels awful.
- Light is the strongest signal. Bright light in the morning and dim light for the hour before bed shift your body clock more reliably than timing alone.
- Caffeine has a long tail. Its half-life is around five to six hours, so an afternoon coffee is still measurably present at bedtime.
This page is a scheduling aid, not medical advice. Persistent trouble sleeping, loud snoring with daytime exhaustion, or falling asleep involuntarily during the day are worth discussing with a doctor.
Questions
Why 90 minutes?
A full sleep cycle — light, deep, then REM sleep — averages about 90 minutes in adults. Waking at the end of a cycle rather than mid-deep-sleep is what tends to reduce grogginess.
How accurate is the 90-minute figure?
It is an average. Real cycles range from roughly 70 to 120 minutes and lengthen through the night, so treat the suggested times as a starting point and adjust the cycle length setting to what works for you.
How long should it take to fall asleep?
Ten to twenty minutes is typical for a healthy adult; the default here is 15. Consistently falling asleep in under five minutes usually signals sleep deprivation.
How many cycles should I aim for?
Five or six — that is 7.5 to 9 hours — matches the sleep duration most adults need. Four cycles (6 hours) is a workable minimum for the occasional short night, not a routine.