Breakdown
How it works
A full night is 5–6 cycles of about 90 minutes. Count back from your alarm in 90-minute steps and add the 10–20 minutes it takes to fall asleep. The times shown are cycle boundaries; pick the one that gives 7.5–9 hours.
Formula
Bedtime = wake time − (cycles × 90 min) − time to fall asleep
Wake time = bedtime + time to fall asleep + (cycles × 90 min)
Worked example
Alarm at 06:30, 15 minutes to fall asleep: sleep at 21:15 (6 cycles), 22:45 (5) or 00:15 (4).
Frequently asked questions
Are cycles exactly 90 minutes?
They range 70–120 minutes and lengthen through the night; the 90-minute figure is an average. Consistency of schedule matters more.
How much sleep do adults need?
7–9 hours. Under 6 regularly impairs performance as much as mild intoxication.
Does it work for naps?
Nap 20 minutes (no deep sleep) or a full 90-minute cycle; 30–60 minutes causes sleep inertia.