Calculate the best bedtime or wake-up time based on 90-minute sleep cycles. Find out when to sleep or wake up to feel fully rested and avoid grogginess. Designed specifically for developers and professionals, this free online calculator runs entirely in your browser to process tasks quickly and securely. Enjoy unlimited usage with no data tracking, no registration required, and an entirely ad-free experience.
Assumes ~14 min to fall asleep after you go to bed.
2:26 PM
6 cycles · 9 hours of sleep
3:56 PM
5 cycles · 7.5 hours of sleep
5:26 PM
4 cycles · 6 hours of sleep
6:56 PM
3 cycles · 4.5 hours of sleep
Sleep happens in repeating 90-minute cycles. Each cycle moves through light sleep, deep sleep, and REM (dream) sleep. Waking up at the end of a complete cycle — rather than in the middle — helps you feel alert and refreshed.
Most adults need 5–6 cycles (7.5–9 hours). Waking mid-cycle causes sleep inertia — the groggy feeling you get when an alarm cuts deep sleep short.
Calculate the best time to go to sleep or wake up based on natural 90-minute sleep cycles. Avoid grogginess by timing your alarm to the end of a complete cycle.
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