Wake up refreshed.

Sleep runs in cycles of roughly 90 minutes. Waking at the end of a cycle, when sleep is lightest, tends to feel less groggy than waking from deep sleep. The calculator adds your time to fall asleep (14 minutes by default) plus whole 90-minute cycles to your bedtime, or subtracts them from your wake-up time.

For 5 cycles (7.5 hours of sleep) go to bed at 11:16 PM; for 6 cycles (9 hours) at 9:46 PM; for 4 cycles (6 hours) at 12:46 AM. These include 14 minutes to fall asleep.

Most adults need 5 cycles (about 7.5 hours) or 6 cycles (about 9 hours). The AASM and Sleep Research Society recommend at least 7 hours a night for adults. 4 cycles (6 hours) is an occasional fallback, not a habit.

More total sleep beats perfect cycle timing. 7+ hours is the recommendation for adults. Use cycle timing to pick a wake-up time within that, e.g. 7.5 hours instead of 7.

No. 90 minutes is an average; individual cycles range from about 70 to 120 minutes and change through the night. Treat results as a best estimate and adjust the fall-asleep time if you consistently wake up groggy.

Common reasons: waking mid-cycle from deep sleep, late caffeine or alcohol, an irregular schedule, or a sleep disorder such as sleep apnea. If tiredness persists despite enough sleep, talk to a doctor.