How long a sleep cycle really is
The Sleep Foundation describes four to six cycles a night averaging about 90 minutes, but notes that the first cycle is often the shortest at 70–100 minutes, while later cycles tend to run 90–120 minutes. Deep sleep (N3) is concentrated early in the night, and REM periods get longer toward morning.
So "90 × 5" is a convenient approximation. A calculator gives you a reference point for waking up; it cannot promise you will be in light sleep at that exact minute.
What 10 minutes does
Recommended bedtime for five cycles if you must be up at 7:00 and take 15 minutes to fall asleep:
| Cycle length | Sleep for 5 cycles | Bedtime |
|---|---|---|
| 80 min | 6h 40m | 00:05 |
| 85 min | 7h 05m | 23:40 |
| 90 min | 7h 30m | 23:15 |
| 95 min | 7h 55m | 22:50 |
| 100 min | 8h 20m | 22:25 |
Same "five cycles", but 1 hour 40 minutes apart between 80 and 100. If calculator times never feel right, the calculator is probably not wrong; its cycle-length assumption just isn’t yours.
Estimating your own cycle
- Collect alarm-free mornings — on days off, note when you fell asleep and when you woke up naturally.
- Divide by the number of cycles — 7h 50m (470 min) and a natural wake-up suggests five cycles of about 94 minutes.
- Test it for a week or two — use that value, rate your mornings, and nudge it by 5 minutes if you still wake up groggy.
This is only an estimate. Sleep stages can’t be measured precisely without a sleep study.
Changing the cycle length in the app
The Android app Sleep Calculator starts at 90 minutes, but lets you change both the cycle length and the time it takes you to fall asleep.
Set your cycle length
Tap the gear icon, then the Sleep cycle length button to step through 80, 85, 90, 95 and 100 minutes. Time to fall asleep goes from 0 to 45 minutes. Values like 75, 105 or 110 minutes are in "Premium presets", unlocked by watching an optional rewarded ad.
Recalculate
On the Calculator tab, choose When to sleep? and enter your wake time. Bedtimes for 6, 5, 4 and 3 cycles are recalculated with your cycle length; 5 and 6 cycles are marked "Best".
Log each morning and compare
Record when you fell asleep, when you woke up and a star rating in the Log tab, so you can compare which cycle setting gave you better mornings.
FAQ
Is there a way to measure my sleep cycle exactly?
Sleep stages are measured with an overnight sleep study (polysomnography). Wearables only estimate them. This app calculates; it does not measure.
After changing the cycle length, the bedtimes got earlier.
A longer cycle means more total sleep for the same number of cycles, so bedtime moves earlier, as in the table above. If it is too early, look at the 4-cycle option or revisit the value.
Sleep Calculator
No sign-up, and your sleep log stays on your phone. Free on Android.