Sleep Cycle Calculator

The arithmetic here is deliberately blunt: one cycle is treated as exactly ninety minutes, your chosen cycle count is multiplied by that constant, the minutes you expect to spend awake before drifting off are added on top, and the whole block is subtracted from the alarm time you typed. With the standard settings — a seven o'clock alarm, fifteen minutes to fall asleep, five cycles — the target is a quarter past eleven at night. Two things about the results are worth understanding before you trust them. First, the Total Sleep figure counts cycles only, so at five cycles it reads seven and a half hours even though the plan actually keeps you in bed for seven hours and forty-five minutes; the falling-asleep allowance is time in bed, not sleep.

Second, the three alternative rows are hard-wired to four, five and six cycles and are recalculated on every run no matter which target you selected, which is why one of them always duplicates the headline bedtime while the other two show you the hour and a half either side. Lengthening the falling-asleep estimate slides all four times earlier together, since it is added to every variant. What the page cannot know is that ninety minutes is an average rather than a law: real cycles run roughly seventy to a hundred and twenty minutes and tend to stretch as the night goes on, so treat the printed minute as the centre of a window. Anything past midnight is wrapped around to the previous evening automatically.