Bedtimes for a wake-up time, or wake-up times for a bedtime, counted in 90-minute sleep cycles plus the time it takes to fall asleep.
By AdminDrafted with AI writing toolsUpdated Checked examples: 5
To wake at 07:00 after 5 full cycles (7 h 30 min of sleep), get into bed at 23:15.
| Cycles | Sleep | Go to bed at |
|---|---|---|
| 6 | 9 h | 21:45 |
| 5 | 7 h 30 min | 23:15 |
| 4 | 6 h | 00:45 |
| 3 | 4 h 30 min | 02:15 |
Clock arithmetic wraps around midnight.
The calculator counts back from a wake-up time, or forward from a bedtime, in whole sleep cycles plus the time it takes to fall asleep: bedtime = wake-up time − cycles × cycle length − time to fall asleep. Each night's sleep runs through non-REM stages and REM sleep in repeating cycles; the first lasts about 70 to 100 minutes and later ones about 90 to 120, so the default cycle is 90 minutes.
People use it to choose a bedtime or an alarm time that falls between cycles. With the defaults, waking at 07:00 after 5 cycles and 15 minutes to fall asleep means getting into bed at 23:15, for 7 h 30 min of sleep. The table lists the times for 3 to 6 cycles.
Real cycles vary from night to night, so the times are estimates. The American Academy of Sleep Medicine recommends at least 7 hours a night for adults, which 5 or more 90-minute cycles provide and 4 cycles (6 hours) do not.
Count back from your wake-up time in 90-minute sleep cycles, then subtract the time you take to fall asleep. To wake at 07:00 after five cycles (7 h 30 min of sleep) with 15 minutes to fall asleep, get into bed at 23:15. Six cycles move bedtime to 21:45; four cycles, to 00:45.
Bedtime = wake-up time − cycles × cycle length − time to fall asleep. For the example: 5 × 90 + 15 = 465 minutes, and 07:00 minus 7 h 45 min is 23:15 the evening before.
To find a wake-up time from a bedtime, add instead. Getting into bed at 23:30, five cycles plus 15 minutes end at 07:15.
The time to fall asleep counts once, at the start of the night. Sleep time and time in bed therefore differ: 7 h 30 min asleep is 7 h 45 min in bed.
With 90-minute cycles and 15 minutes to fall asleep:
If you usually take 30 minutes to fall asleep, move each time 15 minutes earlier.
About 90 minutes on average, but it varies. Each cycle runs through non-REM stages and REM sleep (NIH); the first lasts about 70 to 100 minutes and later ones about 90 to 120 (Carskadon and Dement).
REM sleep first appears about 90 minutes after falling asleep (NINDS).
Real cycles also vary from night to night. If yours average 100 minutes, five of them end 50 minutes after five counted 90-minute cycles.
Adults need at least 7 hours a night on a regular basis, according to the American Academy of Sleep Medicine and Sleep Research Society. Five 90-minute cycles give 7 h 30 min; four give 6 hours, which falls short.
Children and teenagers need more. The AASM pediatric recommendations are 9 to 12 hours per 24 hours for ages 6 to 12 and 8 to 10 hours for ages 13 to 18. A 15-year-old who gets up at 06:30 needs six cycles, so bed at 21:15.
Total sleep matters more than landing exactly on a cycle boundary. A schedule that wakes you between cycles but leaves you with 6 hours is still short of the adult minimum.
It can reduce grogginess. Sleep inertia is strongest after waking from deep slow-wave sleep, weakest after light stage 1 or 2 sleep, and usually fades within 30 minutes without major sleep loss (Tassi and Muzet, 2000).
A clock calculation can only estimate where cycles end; locating the real boundaries takes measuring your sleep stages.
Hours are decimal: 7.50 hours is 7 h 30 min, not 7 h 50 min.
| Scenario | Suggested clock time | Sleep (hours) | Time in bed (hours) |
|---|---|---|---|
| Default | 23:15 | 7.50 | 7.75 |
| Six-cycle example | 20:45 | 9.00 | 9.25 |
| Forward midnight | 07:15 | 7.50 | 7.75 |
The caffeine half-life calculator estimates how much caffeine is left at bedtime. For plain elapsed-time arithmetic between two clock times, use the time duration calculator.
Checked against: 07:00 − (15 + 5 × 90) min = 23:15 the evening before
Checked against: 06:00 − 555 min = 20:45
Checked against: 23:30 + 465 min = 07:15 next day
Checked against: 00:30 + 360 min
About 90 minutes on average. The US Institute of Medicine's 2006 report on sleep disorders, citing Carskadon and Dement, gives 70 to 100 minutes for the first cycle of non-REM and REM sleep and 90 to 120 minutes for later cycles. The National Institute of Neurological Disorders and Stroke says REM sleep first occurs about 90 minutes after falling asleep. You can change the cycle length under More options.
At least 7 hours a night on a regular basis, according to the 2015 consensus statement of the American Academy of Sleep Medicine and the Sleep Research Society. Regularly sleeping less is linked to weight gain, diabetes, high blood pressure, heart disease, stroke, depression and a higher risk of death. Five 90-minute cycles give 7 h 30 min; four give 6 hours, which falls short.
The American Academy of Sleep Medicine's 2016 consensus recommends 8 to 10 hours per 24 hours for teenagers aged 13 to 18 and 9 to 12 hours for children aged 6 to 12. For a 15-year-old who must get up at 06:30, six 90-minute cycles (9 hours) plus 15 minutes to fall asleep put bedtime at 21:15.
It can, but the timing is an estimate. Grogginess on waking, called sleep inertia, is strongest after waking from deep slow-wave sleep, weakest after stage 1 or 2 sleep and in between after REM, and without major sleep loss it rarely lasts beyond 30 minutes (Tassi and Muzet, Sleep Medicine Reviews, 2000). Because real cycles run 70 to 120 minutes, five counted 90-minute cycles can end half an hour or more away from your actual cycle boundary.
30 minutes or less. The National Sleep Foundation lists falling asleep within 30 minutes as one of four signs of good sleep quality, from its 2017 expert consensus (Ohayon et al., Sleep Health). The calculator's default is 15 minutes; enter your usual time, because every extra 15 minutes moves the suggested bedtime 15 minutes earlier.
For information only, not medical advice. These formulas describe populations, not you; talk to a clinician before acting on a result.
This calculator includes 5 worked examples with independently sourced answers. They run in the test suite, and you can run them here too.