Sleep Calculator
Last updated: 2026-06-25
Sleep repeats in roughly 90-minute cycles, so aiming for multiples of 90 minutes (6, 7.5 or 9 hours) and waking at the end of a cycle leaves you more refreshed.
This calculator suggests a bedtime from your wake-up time, or a wake-up time from your bedtime, in 90-minute cycles (including 15 minutes to fall asleep).
Calculate bedtime or wake-up time
Recommended bedtime
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Please enter a valid time.
How to use it
- Choose direction — wake-up time → bedtime, or bedtime → wake-up time.
- Enter the time — enter the reference time. Time to fall asleep (default 15 minutes) is included.
- View result — press Calculate to see several recommended times based on 90-minute cycles.
90-minute sleep cycles and sleep timing
Sleep flows through light sleep (stages 1–2), deep sleep (stages 3–4) and REM in cycles of about 90 minutes, repeating 4–6 times a night. Waking from deep sleep leaves you groggy, but waking at the end of a cycle, during light sleep, feels more refreshing for the same total sleep.
Bedtime = wake-up time − (90 min × cycles) − time to fall asleep
Example: wake at 7:00, 5 cycles (7.5 h) + 15 min → bedtime 11:15 PM
Total sleep by number of cycles
| Cycles | Sleep | Best for |
|---|---|---|
| 6 cycles | 9 hours | Teens, recovery |
| 5 cycles | 7.5 hours | Standard adult recommendation |
| 4 cycles | 6 hours | Minimum recommended |
| 3 cycles | 4.5 hours | Insufficient, occasional only |
Tips for better sleep
- Going to bed and waking at the same time each day stabilizes your sleep cycle.
- Avoid phones and bright screens for an hour before bed (blue light suppresses melatonin).
- Avoid caffeine late in the day, and keep your bedroom dark and cool.
- Cycle length varies by person, so treat the suggested times as a reference.
If you need more date/time math, try the Date Calculator; for health, the Calorie Calculator.
Frequently asked questions (FAQ)
What is a 90-minute sleep cycle?
Sleep moves through light sleep, deep sleep and REM in cycles of about 90 minutes. Waking at the end of a cycle, during light sleep, leaves you feeling more refreshed even for the same total sleep. So it helps to aim for multiples of 90 minutes (4.5, 6, 7.5 or 9 hours).
How many hours should an adult sleep?
Adults are generally advised to get 7–9 hours. In 90-minute cycles that's 5 cycles (7.5 hours) or 6 cycles (9 hours). Aim for at least 4–5 cycles (6–7.5 hours).
Why is time-to-fall-asleep added?
It usually takes 10–20 minutes to actually fall asleep after lying down. This calculator adds a default 15 minutes so it can recommend the time you should actually be in bed.
Are these recommended times accurate?
Cycle length varies between people, from about 80 to 110 minutes. 90 minutes is an average, so treat the suggestions as a reference. Try them for a few days to find the cycle that leaves you most refreshed.
How long should a nap be?
A 10–20 minute nap, before you enter deep sleep, helps you wake refreshed. Napping over 30 minutes can leave you groggier, and late-afternoon naps can disrupt night sleep.
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Last updated: 2026-06-25