Why does the calculator use 90-minute cycles?
A full sleep cycle averages 90 minutes across light, deep and REM stages. Waking at the end of a cycle, rather than mid-deep-sleep, is what reduces grogginess.
Calculate the best time to sleep and wake up based on 90-minute sleep cycles. Wake up feeling refreshed by completing full cycles.
Bedtime = wake time − (90 min × cycles) − 15 min to fall asleep. Target 5–6 cycles (7.5–9 hours).
To wake at 6:30 am after five cycles, fall asleep by 11:00 pm — so get into bed around 10:45 pm. Six cycles means a 9:15 pm lights-out.
A full sleep cycle averages 90 minutes across light, deep and REM stages. Waking at the end of a cycle, rather than mid-deep-sleep, is what reduces grogginess.
For almost no one. Four cycles (6 hours) leaves most adults with measurable cognitive deficits; adults need 7–9 hours, and short-sleeper genetics affect well under 1% of people.
The 15–30 minutes of impaired alertness after waking. It is worst when an alarm interrupts deep sleep, which is exactly what cycle-aligned bedtimes are designed to avoid.