Sleep Calculator
Find the best time to go to sleep or wake up based on 90-minute sleep cycles, so you wake at the end of a cycle rather than in the middle of deep sleep.
FreeNo sign-upPrivate — runs in your browser, nothing is uploaded
Try to fall asleep at one of these times:
Based on 90-minute sleep cycles plus ~15 minutes to fall asleep. Waking at the end of a cycle can feel more refreshing. Sleep needs vary by person — this is general information, not medical advice. Computed in your browser.
Estimate only. This tool is for general information only and is not medical advice or a diagnosis. Results are estimates based on the formula shown. Consult a qualified healthcare professional for guidance about your own situation.
Quick answer
Sleep happens in cycles of about 90 minutes, and waking at the end of a cycle tends to feel more refreshing than waking mid-cycle. To wake at 7:00 AM, good bedtimes are about 9:45 PM (6 cycles, 9 hours) or 11:15 PM (5 cycles, 7.5 hours), allowing ~15 minutes to fall asleep. This calculator lists the best bed or wake times; aim for 5–6 cycles (7.5–9 hours) for most adults.
Formula & method
The tool assumes a 90-minute cycle and about 15 minutes to fall asleep. Working back from a wake time, each suggested bedtime is wake − (15 + 90 × cycles); working forward from a bedtime, each wake time is bedtime + (15 + 90 × cycles), for 3 to 6 cycles. Five or six cycles (7.5–9 hours) is highlighted as the typical adult target.
Examples
- Input
- Wake-up mode, 07:00
- Result
- Bed 9:45 PM (6 cycles)
- Why
- 6 cycles = 9 hours, plus 15 min to fall asleep, before 7:00 AM.
- Input
- Wake at 7:00 AM
- Result
- Bed 11:15 PM (7.5 h)
- Why
- 5 cycles is a common 7.5-hour target.
- Input
- Bedtime mode, 23:00
- Result
- Wake 6:30 AM (5 cycles)
- Why
- 11:00 PM + 15 min + 5×90 min ≈ 6:30 AM.
When to use this tool
- Picking a bedtime to wake up refreshed at a fixed alarm time.
- Choosing when to set an alarm if you're going to bed now.
- Planning sleep around an early start or a nap.
Common mistakes
- Forgetting the ~15 minutes it takes to fall asleep, which shifts the ideal wake time.
- Treating exactly 90 minutes as universal — real cycles vary from roughly 70 to 120 minutes per person and night.
- Chasing cycle timing while ignoring total sleep; most adults need 7–9 hours regardless.
Frequently asked questions
How long is a sleep cycle?
About 90 minutes on average, cycling through light, deep, and REM sleep. Waking at the end of a cycle generally feels better than waking during deep sleep.
How many hours of sleep do I need?
Most adults need 7–9 hours, which is about 5–6 sleep cycles. Teens and children need more. Individual needs vary.
Why add 15 minutes?
It's a typical amount of time to fall asleep (sleep latency). The calculator includes it so the cycle timing lines up with when you actually drift off.
Are sleep cycles exactly 90 minutes?
No. Ninety minutes is an average; real cycles range roughly 70–120 minutes and change through the night, so treat the times as guides.
Is this medical advice?
No. It's general information based on average cycles. For ongoing sleep problems, consult a healthcare professional.
Is anything uploaded?
No — the times are computed in your browser.
Sources & references
External references open in a new tab. We are independent and not affiliated with these organizations.
Disclaimer
This tool is for general information only and is not medical advice or a diagnosis. Results are estimates based on the formula shown. Consult a qualified healthcare professional for guidance about your own situation.
- ✓ Free to use
- ✓ No sign-up required
- ✓ Runs entirely in your browser — nothing is uploaded.
- ✓ Formula and method shown above
Provided “as is” for general information only — results may be inaccurate, so verify before you rely on them. No warranty; use at your own risk.
Built and reviewed by HIFreeTools against the formula shown above and any authoritative references cited on this page. See our methodology and editorial standards.
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