Sleep calculator
Wake at the end of a 90-minute cycle, not in the middle.
- Instant
- Free
- Private (processed locally)
- No sign-up
Wake up at the right moment, not the wrong one
How you feel at wake-up depends less on the number of hours than on the point in the cycle when the alarm rings. This tool aligns your night with 90-minute cycles: enter your bedtime to get the good wake times, or your imposed wake time to know when to turn off the light.
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Pick the direction
“I go to bed at…” or “I must wake up at…” — the calculation flips automatically.
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Set the time
Or click Now if you are heading to bed right away.
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Aim for 5 or 6 cycles
The recommended (violet) cards correspond to 7.5 and 9 hours of actual sleep.
Options at a glance
| Cycles | Actual sleep | For whom |
|---|---|---|
| 3 | 4 h 30 | Emergency night — long nap or crunch |
| 4 | 6 h | Occasional minimum, not to repeat |
| 5 | 7 h 30 | The target for most adults ★ |
| 6 | 9 h | High need, recovery, teenagers ★ |
These times are averages: cycles vary from 80 to 110 minutes between individuals. Watch your wake-ups for a week and adjust. For persistent sleep trouble, talk to a doctor.
Frequently asked questions
What is a sleep cycle?
A night is organized in cycles of about 90 minutes chaining light sleep, deep sleep, then REM (dreams). Waking at the end of a cycle — during light sleep — is far easier than in the middle of deep sleep, hence the “natural” wake feeling.
How many cycles do I need per night?
Five cycles (7.5 hours of sleep) is the recommended target for most adults; six cycles (9 hours) for high sleep needs. Four cycles (6 hours) works occasionally, but below that sleep debt builds up fast.
Why add 15 minutes?
It is the average adult sleep-onset latency. The tool adds it automatically between bedtime and the start of the first cycle. If you fall asleep faster or slower, shift accordingly.
Why do I feel groggy even after 8 hours?
Eight hours does not land on a cycle boundary: 8 h = 5.3 cycles. The alarm rings in the middle of deep sleep — that is sleep inertia. 7.5 h (5 cycles) or 9 h (6 cycles) often beat 8 h for wake-up clarity.