Checked 15 October 2026. What was checked. How these pages are maintained.
90-minute cycles
A common sleep-hygiene trick is to wake at the end of a ~90-minute cycle instead of in the middle of deep sleep. This page adds 15 minutes to fall asleep, then counts 4–6 cycles (6–9 hours). It is a planning aid, not a diagnosis of insomnia or sleep apnea.
Worked example
To wake at 7:00 after 90-minute cycles, typical bedtimes are about 11:00, 12:30, or 9:30 the night before, plus ~15 minutes to fall asleep. Five cycles is 7.5 hours of sleep.
Cycle length is not exactly 90 minutes for everyone. Shift work, caffeine, and a new baby blow up the chart. This is a schedule sketch, not a sleep clinic. Not medical advice.
Ninety minutes, plus the time it takes to fall asleep
The page treats one cycle as 90 minutes and adds 15 minutes to fall asleep. To be awake at 6:30, five cycles are 5 × 90 + 15 = 465 minutes, which is 7 hours and 45 minutes. Counting back from 6:30 lands at 10:45 p.m. Six cycles are 9 hours and 15 minutes of that same sum, which is 9:15 p.m. Four cycles are 6 hours and 15 minutes, which is 12:15 a.m. The three lines are those three options. The page is not saying you must sleep five cycles. It is showing where the 90-minute boundaries sit relative to the clock time you typed.
The other direction, “I am getting into bed now,” adds the 15 minutes and then the cycles forward and prints a wake time. Cycles are not exactly 90 minutes in a real night. They shorten and lengthen, and the 15 minutes to fall asleep is a guess that is wrong if you are asleep in five minutes or still awake at midnight.
A sketch, not a sleep study
Shift work, caffeine late in the day, a new baby, and a room that is too warm will not honour the grid. Waking at the end of a cycle is a comfort idea, not a treatment for insomnia or sleep apnea. If sleep is a health problem, a clinician is the next step, not a finer grid. The math stays in the browser. Nothing about the time you picked is stored.
Common questions
Do cycles last exactly 90 minutes?
No. They vary. 90 minutes is a popular average.
How much sleep do adults need?
Many adults do well near 7–9 hours. Follow how you feel and a clinician’s advice.