Waking up mid-cycle — especially during deep sleep — tends to produce noticeable grogginess even after a full night’s rest. Our sleep cycle calculator works backward or forward from a target time to suggest bedtimes or wake-up times that land at the natural boundary between cycles.
The math
Sleep progresses through repeating ~90-minute cycles. The calculator adds a 15-minute buffer (a commonly cited average time-to-fall-asleep) before counting whole 90-minute blocks:
Bedtime = wake time − (cycles × 90 minutes + 15 minutes)
Wake time = bedtime + 15 minutes + (cycles × 90 minutes)Example
Want to wake up at 7:00 AM? The calculator suggests four bedtimes, each ending a whole number of complete cycles:
- 9:45 PM → 6 cycles (9 hours)
- 11:15 PM → 5 cycles (7.5 hours)
- 12:45 AM → 4 cycles (6 hours)
- 2:15 AM → 3 cycles (4.5 hours)
Two modes
Switch between “I want to wake up at…” (working backward to find bedtimes) and “I’m going to bed now” (working forward to find wake-up times) depending on which direction your schedule constraint runs.
A caveat on precision
90 minutes is a widely cited average, but individual cycle length genuinely varies — commonly cited estimates range from roughly 70 to 120 minutes, and can shift somewhat night to night for the same person. Treat these suggested times as a useful starting point to experiment with over a few nights, not a precise biological guarantee.