Sleep Cycle Calculator
Sleep moves in roughly 90-minute cycles, and the groggiest mornings come from an alarm landing mid-cycle rather than at its end. This planner works in both directions: sleep right now and it lists the four best wake-up windows over the next nine hours; entering a target wake-up time lists the four bedtimes that line up with a cycle boundary.
Fifteen minutes of fall-asleep time is built in, because "in bed at 11:00" and "asleep at 11:15" are different plans. Six cycles (about 9 hours) is the generous pick, five (7.5 hours) suits most adults, and three cycles (4.5 hours) is the short-night floor — the times update live, are remembered on this device, and the share button packages your plan for whoever needs to be up with you.
How to use
- Choose "sleep now" to see wake-up times from 6 down to 3 cycles, each with its hours of sleep.
- Or switch to "bedtime" mode and enter the time you must wake up — the four cycle-aligned bedtimes appear instantly.
- Share the plan with one tap; the link reopens the same mode and time on the other person's phone.
Frequently asked questions
What time should I wake up if I sleep now?
Take the current time, add about 15 minutes to fall asleep, then add 90-minute cycles: the planner shows wake-ups at roughly 9, 7.5, 6 and 4.5 hours out. Waking at any of those boundaries — especially 5-6 cycles — feels noticeably lighter than waking mid-cycle.
How many sleep cycles do you need?
Most adults do best on 5-6 cycles (7.5-9 hours); 4 cycles (6 hours) works occasionally, and 3 cycles (4.5 hours) is the short-night minimum, not a routine. The planner lists all four so you can pick honestly for tonight.
Why add 15 minutes to fall asleep?
Sleep latency — the average time it takes to drift off once lights are out. Bedtimes here are lights-out times, so the planner subtracts latency from your target wake-up to keep the cycle math honest. If you typically fall asleep faster or slower, mentally shift the results the same way.
Does the 90-minute cycle rule actually work?
It is a useful average, not a biological law — individual cycles run 80-100 minutes and vary night to night. Used as a planning heuristic (which is all this calculator claims), aligning alarms with cycle boundaries reliably reduces grogginess from sleep inertia.