Sleep Calculator — When To Go To Bed
Work back from your alarm in 90-minute sleep cycles to find bedtimes that avoid waking mid-cycle.
- Recovery
- 2 cited sources
- Runs in your browser
Suggested bedtimes
- 21:456 cycles · 9 h asleep
- 23:155 cycles · 7.5 h asleep · suits most adults
- 00:454 cycles · 6 h asleep
Times include 15 minutes to fall asleep and assume 90-minute cycles. Real cycles run roughly 70–120 minutes and vary between people — treat these as starting points and adjust to how you feel on waking.
Methodology
The calculator counts backwards from your alarm:
bedtime = wake time − (cycles × 90 minutes) − 15 minutes to fall asleep
Sleep is organised in cycles that pass through light sleep, deep slow-wave sleep and REM. A full cycle averages roughly 90 minutes in adults — the physiology literature puts the true range at about 70–120 minutes, lengthening as the night goes on. The 15-minute allowance reflects typical sleep latency: the tool aims to start your first full cycle when you are actually asleep, not when your head hits the pillow.
Options are shown for six cycles (9 hours of sleep), five (7.5 hours) and four (6 hours). Five cycles lands inside the 7–9 hours the National Sleep Foundation's expert panel recommends for most adults (Hirshkowitz et al., 2015); four is a compromise for occasional short nights, not a target.
Interpreting your result
The idea behind cycle-based bedtimes is to avoid the alarm landing in deep sleep, when waking feels most groggy — that heavy-headed state is called sleep inertia. Waking near a cycle boundary, when sleep is lightest, tends to feel easier even at the same total duration.
Use the suggested times as starting points and run a simple experiment: hold one bedtime for a week and note how waking feels. Consistency is the real lever — a regular schedule does more for sleep quality than precision-timed bedtimes ever will.
Limitations
- Ninety minutes is an average. Your cycles may run 75 minutes or 110, and they lengthen across the night, so no calculator can time your cycles exactly.
- Sleep latency varies day to day with caffeine, stress, light and exercise timing; the fixed 15-minute allowance is a convenience.
- Alcohol, shift work and jet lag restructure sleep architecture entirely.
- Persistent difficulty falling asleep, staying asleep, or heavy daytime sleepiness warrants a conversation with your GP rather than a schedule tweak.
Sources
Pull the receipts — 2 cited sources
Frequently asked questions
- How many sleep cycles do I need?
- Five 90-minute cycles (7.5 hours of sleep) suits most adults, aligning with the widely recommended 7–9 hours. Four cycles is a short night to use sparingly; six is a full night many people do best on.
- Why 90-minute cycles?
- A full pass through light, deep and REM sleep averages roughly 90 minutes in adults. Waking at the end of a cycle, when sleep is lightest, tends to feel easier than an alarm landing in deep sleep mid-cycle.
- Is everyone's sleep cycle exactly 90 minutes?
- No — real cycles run from about 70 to 120 minutes, vary between people, and lengthen through the night. Treat these bedtimes as sensible starting points and nudge earlier or later based on how you feel on waking.
- Why does the calculator add 15 minutes?
- Most people take around 10–20 minutes to fall asleep, so the tool subtracts an extra 15 minutes so your first full cycle starts once you're actually asleep. If you routinely drop off faster or slower, shift the suggested bedtime accordingly.
- What if I can't fall asleep at the suggested time?
- Consistency beats precision: a regular sleep and wake time, a dark cool room, and winding down without screens matter more than hitting an exact minute. Persistent trouble sleeping is worth discussing with your GP.
Written and reviewed by Mathew Beale — MSc Biotechnology, University of Reading.
Last reviewed:
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