The Sleep Calculator suggests the best times to go to bed or wake up, based on 90-minute sleep cycles and the time it takes to fall asleep.
To wake up at 6:00 AM, go to bed at 8:45 PM or 10:15 PM.
| 6 sleep cycles (9.0 hours of sleep) | 8:45 PM |
| 5 sleep cycles (7.5 hours of sleep) | 10:15 PM |
| 4 sleep cycles (6.0 hours of sleep) | 11:45 PM |
| 3 sleep cycles (4.5 hours of sleep) | 1:15 AM |
During the night the brain moves through cycles of light sleep, deep sleep and REM sleep, each lasting roughly 90 minutes (70–120 for different people). Waking at the end of a cycle, during light sleep, usually feels easier than being woken from deep sleep.
For a wake-up time, the calculator counts back whole sleep cycles plus the time it takes to fall asleep. For a bedtime, it counts forward.
Worked example (defaults): to wake at 6:00 AM with 90-minute cycles and 15 minutes to fall asleep, go to bed at 8:45 PM (6 cycles, 9 hours) or 10:15 PM (5 cycles, 7.5 hours).
Adults generally need 7–9 hours, teenagers 8–10 and school-age children 9–12. Consistent bed and wake times, a dark cool room, limiting caffeine and screens before bed, and daylight in the morning all help.
Is 90 minutes exact? No; adjust the cycle length if you often wake groggy.
Do naps follow the same rule? A 20-minute nap or a 90-minute full-cycle nap avoids grogginess.
When should I see a doctor? If you snore loudly, stop breathing at night or feel tired despite enough sleep.