When should you go to bed?

A sleep cycle averages 90 minutes and runs through four stages: light sleep, deeper light sleep, slow-wave (deep) sleep, and REM. Most adults complete five cycles in a typical night — that is the 7.5-hour figure you have probably read about. Teens average six. Seniors land at five.

The trick is to wake at the end of a cycle, not the middle of one. We add the time it takes you to fall asleep (most people land at 10–20 minutes), then count 90-minute cycles back from your wake time. The middle option in the result is the recommended one. The earlier option is "good" — one more cycle of sleep, but an earlier wind-down. The later option is "minimal" — less sleep but lets you stay up later.

Three caveats. One, 90 minutes is an average; your cycles could run anywhere from about 80 to 110 minutes night to night. Two, the first cycle is often shorter than the last. Three, this is a heuristic — not a sleep diagnosis. If you wake up exhausted regardless of when you go to bed, see a sleep clinician.

Each bedtime the calculator returns is timed so a full number of 90-minute cycles ends right at your wake time, leaving you in light sleep when the alarm goes. More cycles means more total rest, so pick the earliest of these bedtimes you can actually keep; if you cannot make it, the next one is a full cycle later rather than a half-cycle compromise.

The 15-minute offset is the time it takes a typical adult to fall asleep, added so the math starts from sleep onset, not from lights-out. Personal cycle length varies from roughly 80 to 110 minutes, so if you still wake groggy on the suggested schedule, try shifting bedtime 15 minutes earlier and compare how you feel.

Sleep runs in repeating cycles of light sleep, deep slow-wave sleep, and REM. Deep sleep is front-loaded into the first cycles of the night and REM is back-loaded into the last, so the same clock reading can sit in very different stages depending on when you went to bed.

Waking out of deep sleep produces sleep inertia, the heavy, useless grogginess that Trotti's review documents as measurable cognitive impairment lasting up to an hour after the alarm. Eight hours does not divide evenly into 90-minute cycles and often lands mid-cycle, while 7.5 hours (five clean cycles) ends at a boundary, which is why the shorter night can feel sharper.

The single biggest multiplier is a consistent wake time. The cycle math only pays off if your schedule is stable enough for your body clock to anchor to it, so keep the wake time fixed even on weekends and let bedtime flex.

Treat the 90-minute figure as a population average, not a lab measurement. It gets most adults close enough to feel a difference, but it will not out-perform a genuinely consistent routine, and it does nothing about caffeine, a warm room, or a racing mind. Pair it with a wind-down rather than treating it as the whole fix.