Sleep Calculator
Count backwards from your alarm in 90-minute cycles: 21:45, 23:15 or 00:45 for a 07:00 wake-up.
Sleep runs in cycles, not in whole hours
A night is a sequence of sleep cycles of roughly 90 minutes each. Waking at the end of a cycle usually feels better than waking in the middle of one, even with the same total hours — which is why "eight hours" and "seven and a half hours" can feel like different nights.
This calculator counts backwards from the time you need to get up, subtracts the minutes you usually take to fall asleep, and gives you three bedtimes: six cycles, five, and four.
Worked example: up at 07:00
Assuming 15 minutes to fall asleep:
| Bedtime | Sleep | Cycles | Who it suits |
|---|---|---|---|
| 21:45 | 9 h | 6 | Teenagers, athletes, anyone catching up on a debt |
| 23:15 | 7 h 30 | 5 | Most adults — inside the 7–9 hour range |
| 00:45 | 6 h | 4 | Short night; fine occasionally, not as a habit |
Going the other way: if you go to bed at 23:00 and take 15 minutes to fall asleep, five cycles put the alarm at 06:45 and six cycles at 08:15.
Why the answer is not exact
- Cycle length varies. 90 minutes is an average; individual cycles run 80–120 minutes and lengthen towards morning, when REM periods get longer.
- Falling asleep time is an estimate. 15 minutes is typical; if you lie awake for 40, the whole schedule shifts. Adjust the field rather than guessing.
- You do not wake at the same point every night. The last cycle is often cut short by the alarm, which is exactly the cycle this tool tries to protect.
- Total sleep still matters. Hitting a cycle boundary does not compensate for five hours a night across a week.
What actually improves sleep timing
- A fixed wake time, seven days a week. The wake time anchors the circadian rhythm; bedtime follows from sleep pressure.
- Light in the morning, dim light in the last hour before bed. This is the strongest signal available without equipment.
- Caffeine cut-off around early afternoon: the half-life is about five hours, so a 16:00 coffee still has a quarter of its effect at midnight.
- Alcohol is not a sleep aid. It shortens the time to fall asleep and then fragments the second half of the night.
When to stop using a calculator. Loud snoring with pauses in breathing, gasping awake, morning headaches, or falling asleep during the day despite seven hours in bed are reasons to see a clinician, not to shift your bedtime by 15 minutes. This page is arithmetic, not a diagnosis.
Questions
Is 90 minutes the right cycle length for everyone?
No. It is the population average used by every "sleep cycle" calculator. If your own cycles run 100 minutes, the bedtimes here will be early by roughly 10 minutes per cycle — noticeable over five cycles, which is why the fixed wake time matters more than the exact bedtime.
Should I aim for six cycles every night?
Aim for the range your age needs, which for most adults is 7–9 hours — five to six cycles. Six cycles is not a target to beat; consistently short sleep is associated with worse outcomes than consistently adequate sleep.
Does the page remember my times?
No. The calculation runs in your browser and nothing is stored or sent. Reload the page and it returns to the 07:00 example.
All pages
Four calculators and three guides. The cycle constants, the age bands, the debt arithmetic and the nap lengths are the same ones the tools use, printed on the page so the numbers can be checked by hand.