Bedtime calculator
When should you go to bed?
What the calculator is doing
Sleep is not uniform. You move through light sleep, deep sleep and REM in cycles of roughly ninety minutes, and where an alarm lands inside that pattern changes how you feel far more than most people expect. Waking at the end of a cycle, in light sleep, feels survivable. Waking out of deep sleep produces the heavy, disoriented state called sleep inertia — and it can last the best part of an hour.
So the calculator works backwards from your wake time in ninety-minute blocks, adds the time it takes you to fall asleep, and shows you the bedtimes that land you at a boundary. Five or six cycles — seven and a half to nine hours — is where most adults should be aiming.
Read the longer explanation in how sleep cycles decide whether your alarm feels brutal, then set the alarm itself.
Common questions
Is the 90-minute sleep cycle real?
It is a useful average, not a law. Adult sleep cycles typically run between roughly 70 and 120 minutes and they change through the night — early cycles carry more deep sleep, later ones more REM. Treat the calculator as a way to pick a sensible bedtime, not a precision instrument.
Why does it add 15 minutes?
That is sleep latency — the time it takes to actually fall asleep after you lie down. Fifteen minutes is a common healthy average. If you know you take longer, raise the number and the suggested bedtimes shift earlier.
Should I really get up at a cycle boundary if it means less sleep?
Usually not. Total sleep matters more than cycle timing. If the choice is 7.5 hours ending mid-cycle or 6 hours ending neatly, take the 7.5. Cycle timing is a tie-breaker between similar options, not a reason to cut an hour.