Sleep calculator
Enter the time you need to wake up. The calculator counts back in 90-minute sleep cycles and suggests bedtimes for six, five, and four cycles, plus time to fall asleep.
Use it to pick a bedtime that lets you wake at the end of a sleep cycle, not mid-cycle.
Bedtime — 6 cycles (9h)
21:45
- Bedtime — 5 cycles (7.5h)
- 23:15
- Bedtime — 4 cycles (6h)
- 00:45
The result updates as you type. The headline is the bedtime for six cycles (about nine hours); the others are five and four cycles.
How does it work?
Waking at the end of a cycle, rather than mid-cycle, tends to feel more refreshing. A cycle averages 90 minutes but varies by person; treat these as guides.
Sleep cycle formula
- wake
- Your wake-up time.
- c
- Number of 90-minute sleep cycles.
- 15
- Minutes allowed to fall asleep.
To wake at 07:00, six cycles means bed at 07:00 − (6×90 + 15) = 21:45; five cycles is 23:15.
Method & sources
A sleep cycle is treated as 90 minutes. 15 minutes are allowed to fall asleep before the first cycle. Bedtimes wrap across midnight as needed.
Sources
Where this method comes from — use these references to understand the formula, assumptions, and limits.
- Sleep cycles and sleep hygiene — U.S. National Heart, Lung, and Blood Institute, verified 2026-06-10
How we calculate
- A sleep cycle is treated as 90 minutes.
- 15 minutes are allowed to fall asleep before the first cycle.
- Bedtimes wrap across midnight as needed.
- Cycle length varies by person; these are guides.
Rounding
Bedtimes are shown as 24-hour times. The calculation uses whole minutes.
What this calculator does
Sleep runs in cycles of roughly 90 minutes. Waking at the end of a cycle tends to feel easier than being jolted awake mid-cycle. This calculator counts back from your wake time in whole cycles, adding 15 minutes to fall asleep, to suggest bedtimes.
How to use it
- Enter the hour you need to wake up (0–23).
- Enter the minute.
- Read the bedtime for six, five, or four cycles.
- Pick the one that gives you enough sleep.
A worked example
To wake at 07:00, six cycles means going to bed at 21:45, five cycles at 23:15, and four cycles at 00:45 — each including 15 minutes to drift off.
How much sleep do you need?
Most adults do well on five to six cycles (7.5–9 hours). Four cycles (6 hours) is a minimum that suits some people short-term. Pick the option that leaves you rested over several days.
Common mistakes
- Entering the wake time in 12-hour form without adjusting AM/PM.
- Assuming exactly 90-minute cycles — yours may differ.
- Chasing cycle timing while ignoring total sleep.
When it's useful
Planning a bedtime around a fixed alarm, an early start, or shifting your schedule and wanting to wake more refreshed.
FAQ
- How are the bedtimes calculated?
- From your wake time, each option subtracts a whole number of 90-minute cycles plus 15 minutes to fall asleep.
- Why aim for the end of a cycle?
- Waking from light sleep at the end of a cycle usually feels easier than being woken from deep sleep mid-cycle.
- Is every cycle exactly 90 minutes?
- No. Ninety minutes is an average; real cycles vary by person and night. Treat the times as a guide.
- How many cycles should I aim for?
- Five to six cycles suits most adults. Four is a short-term minimum. Consistency matters more than hitting an exact number.
- Does the 15 minutes to fall asleep matter?
- It nudges the bedtime earlier so the cycles start once you're actually asleep. Adjust if you usually drift off faster or slower.
- Can I share a calculation?
- Yes. Use Share to copy a link that reopens the calculator with the same wake time.
Related calculators
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<p>Calculator from <a href="https://wisecalcs.com/en/health/sleep-calculator">WiseCalcs</a></p>