Sleep calculator (when to sleep or wake up)
Find the best time to sleep or wake up, based on 90-minute sleep cycles.
Each cycle is ~90 min; waking between cycles feels better. Includes ~15 min to fall asleep.
How the calculation works
Sleep happens in cycles of about 90 minutes. Waking at the end of a cycle (not mid-cycle) reduces grogginess. We count multiples of 90 min from your target time, plus ~15 min to fall asleep.
Examples
- To wake at 6am, good bedtimes: ~9:15pm, 10:45pm or 12:15am (90-min cycles + 15 to fall asleep).
- If going to bed now, aim to wake 6 cycles later (~9h15 total).
Frequently asked questions
How many hours should I sleep?
Adults: 7 to 9 hours, i.e. about 5 to 6 full cycles.
Why do I wake tired after 8 hours?
The alarm likely rang mid-cycle (deep sleep). Waking at the end of a cycle, even a bit earlier, usually feels better.
Are the 90 minutes exact?
It is an average — cycles range ~80 to 110 min between people. Use the times as a guide, not a strict rule.
How many hours of sleep are ideal?
For adults, 7 to 9 hours — five or six complete cycles (7h30 or 9h). Teenagers need 8 to 10 hours.
Is a sleep cycle always 90 minutes?
No, it is an average. In practice it ranges from 80 to 110 minutes across people and across the night.
Why does short sleep hurt memory so much?
Because REM sleep, tied to memory consolidation, is concentrated in the second half of the night. Cutting the end removes REM disproportionately.
How late can I drink coffee?
Caffeine has a half-life of about 5 hours: a 4pm coffee still has half its dose active at 9pm. Avoid it 6–8 hours before bed.
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Updated on June 18, 2026 · by Rafael Rossi · Methodology & sources