Sleep Calculator
Find the best bedtime for a target wake-up time, or the best wake-up time for a target bedtime, based on complete 90-minute sleep cycles.
How It's Calculated
Formula
\text{time} = \text{target} \pm (n \times 90\text{ min} + 14\text{ min}), \quad n \in \{3,4,5,6\}Sleep happens in cycles that average about 90 minutes each, moving through lighter and deeper stages. Waking up in the middle of a cycle (during deep sleep) tends to feel groggier than waking up near the end of one, between cycles. This calculator suggests times that land on whole numbers of sleep cycles — 3, 4, 5, or 6 cycles, corresponding to about 4.5, 6, 7.5, and 9 hours of sleep. It also adds a 14-minute buffer for the average time it takes to fall asleep, based on typical sleep-onset research. If you enter a target wake-up time, it counts backward to suggest bedtimes; if you enter a target bedtime, it counts forward to suggest wake-up times. This is a scheduling estimate based on average cycle length, not a medical or diagnostic tool — actual sleep cycles vary by person and by night.
Worked Examples
Target wake-up time: 7:00 AM
- 6 cycles: 6 x 90 + 14 = 554 minutes before 7:00 = 9:46 PM bedtime
- 5 cycles: 5 x 90 + 14 = 464 minutes before 7:00 = 11:16 PM bedtime
- 4 cycles: 4 x 90 + 14 = 374 minutes before 7:00 = 12:46 AM bedtime
Target bedtime: 11:00 PM
- 3 cycles: 3 x 90 + 14 = 284 minutes after 11:00 PM = 3:44 AM wake-up
- 5 cycles: 5 x 90 + 14 = 464 minutes after 11:00 PM = 6:44 AM wake-up
Frequently Asked Questions
Why 90 minutes per sleep cycle?
A full sleep cycle — moving through light sleep, deep sleep, and REM sleep — averages about 90 minutes in adults. Waking up at the end of a complete cycle, rather than partway through one, is generally associated with feeling more rested.
Where does the 14-minute buffer come from?
It's a commonly cited average for how long it takes a person to fall asleep after getting into bed. This calculator adds it so the suggested times account for time spent falling asleep, not just time spent asleep.
Why does it suggest a range of times instead of one answer?
Everyone's ideal amount of sleep differs, and cycle length varies slightly night to night. Showing several whole-cycle options (4.5 to 9 hours) lets you pick the one that best fits your schedule and typical sleep needs.
Is this based on medical advice?
No. This is a general scheduling estimate based on average sleep cycle length, not a personalized medical recommendation. If you have ongoing sleep concerns, consult a healthcare provider.