Pick the time you need to wake up and get four science-flavored bedtimes, or find out when to set your alarm if you fell asleep right now. Based on 90-minute sleep cycles, not the mythical “eight hours exactly.”
Sleep runs in cycles, not hours. Each cycle sweeps through light sleep, deep sleep, and REM over roughly 90 minutes, and the way you feel in the morning depends heavily on which part of the cycle you wake up in. Get pulled out of deep sleep by your alarm and you experience sleep inertia — that heavy, disoriented grogginess that takes an hour to shake. Wake in the shallows between two cycles and you open your eyes almost refreshed, even if the total was seven and a half hours instead of exactly eight.
That’s the logic this calculator uses. Working backwards from your wake-up time, it offers bedtimes spaced one full cycle apart — six cycles (about 9 hours and 15 minutes in bed), five (7h 45m), four (6h 15m), and three (4h 45m) — each padded with fifteen minutes to actually fall asleep. Six and five cycles are the recommendations for most adults; four is a decent floor if you’re short on time; three is flagged as survival mode because no amount of cycle math makes four hours forty-five a full night’s rest. Going the other direction, if you’re heading to bed right now, it flips the math and gives you four candidate alarm times.
A few honest caveats: cycle length varies from person to person (anywhere from 80 to 110 minutes), the fifteen-minute drift-off estimate is an average, and sleep debt from previous nights can’t be fixed with one well-timed bedtime. Consistency beats precision — going to bed and waking at the same times daily does more for how you feel than hitting a perfect cycle boundary. Use the results as targets, not promises, and let a week of experimentation tell you which count of cycles actually leaves you functional.
Nothing here leaves your browser: the math runs locally, your bedtime habits are not recorded, and there is no account, no tracking, no newsletter. Pick a time, get your targets, go to sleep.