What time should you actually go to bed?

Why 90-minute sleep cycles?

A full sleep cycle — moving through light sleep, deep sleep, and REM sleep — averages about 90 minutes, though it varies somewhat by individual and by night. Waking up in the middle of a cycle, particularly during deep sleep, tends to produce more grogginess than waking up between cycles, which is the idea behind timing sleep or wake-up around cycle boundaries rather than an arbitrary duration.

Why is there a 15-minute buffer added?

Most people don't fall asleep the instant they lie down — the buffer accounts for typical time-to-fall-asleep (sleep latency), so the cycle count lines up with when you're actually asleep, not just when you got into bed. Adjust it above if you know your own latency tends to be longer or shorter.

Is this based on real sleep tracking, or a fixed assumption?

It's a fixed 90-minute average, not a measurement of your actual sleep architecture — real cycle length varies by person, by age, and night to night, and only a sleep study or a consumer sleep tracker analyzing your actual data can measure yours specifically. This calculator is a reasonable general planning heuristic, not a substitute for that kind of measurement.

Worked example: waking up at 7:00 AM with a 15-minute fall-asleep buffer: going to bed at 11:15 PM gives 5 full cycles (7.5 hours of sleep), while 9:45 PM gives 6 full cycles (9 hours).