Sleep runs in repeating 90-minute cycles of light sleep, deep sleep and REM. Waking at the end of a cycle — rather than in the middle of deep sleep — tends to feel far more refreshing, even on slightly less sleep. This calculator counts whole cycles backward from your wake time (or forward from your bedtime), adds about 15 minutes to fall asleep, and suggests the times that land you on a cycle boundary.
Reviewed: June 20, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: the 90-minute sleep-cycle model, recomputed in code. General guidance, not medical advice.
How sleep cycles work
Each 90-minute cycle takes you down into deep sleep and back up toward light sleep and REM. If your alarm goes off during deep sleep you feel groggy (sleep inertia); if it goes off as a cycle ends you feel clearer. So the trick is to schedule sleep in whole cycles. Adding around 15 minutes for the time it takes to actually fall asleep makes the targets realistic.
Worked example — wake at 7:00 AM
Scenario: you need to be up at 7:00 AM and take ~15 minutes to fall asleep.
To wake refreshed at 7:00 AM, the best bedtimes are 9:45 PM (6 cycles) or 11:15 PM (5 cycles) — the two options most adults should target. The 4- and 3-cycle times (12:45 AM and 2:15 AM) are there for short nights. Reverse the logic in bedtime mode: go to bed at 10:30 PM and good wake times are about 6:15 AM (5 cycles) and 7:45 AM (6 cycles).
Frequently Asked Questions
It schedules sleep in whole 90-minute cycles so you wake at a cycle's end, counting from your wake or bed time plus ~15 min to fall asleep.
About 90 minutes (range ~70–120). Five to six cycles a night (7.5–9 hours) suits most adults.
About 9:45 PM (6 cycles) or 11:15 PM (5 cycles), allowing 15 minutes to fall asleep.
Most adults need 7–9 hours (about 5–6 cycles). Teens need more; consistency in timing helps too.
Sleep inertia — waking mid-cycle in deep sleep feels groggy. Waking at a cycle's end usually feels better.