How to calculate sleep
Bedtime = Wake time − (cycles × 90 min) − time to fall asleep
Worked example. Go to bed at 9:45 PM or 11:15 PM: 9 hours in bed for 6 cycles.
Example inputs — I want to wake up at: 07:00; Time to fall asleep: 15 min. Starting values are illustrative.
Other options: 12:45 AM (4 cycles, 6 h)
From George Guju, founder of Calculyn · Educational calculator guidance, not professional advice.
How it works
Sleep runs in roughly 90-minute cycles. Waking at the end of a cycle, rather than in deep sleep, tends to feel easier. Most adults need 5–6 cycles (7.5–9 hours).
Assumptions & limitations
- Results are rounded for display.
Formula and assumptions reviewed October 2026 by the Calculyn team. How we build our calculators
Frequently asked questions
Are sleep cycles always 90 minutes?
They average about 90 minutes but range from roughly 70 to 120 minutes, so treat these times as a guide.