Sleep Calculator
A sleep calculator works backward or forward from a fixed time using average 90-minute sleep cycles, so you wake up between cycles rather than in the middle of deep sleep. Enter a wake-up time to get several candidate bedtimes, or enter a bedtime to get several candidate wake times, each covering a whole number of 4 to 6 sleep cycles plus roughly 14 minutes to fall asleep. Waking at a cycle boundary is a widely used sleep-hygiene heuristic, not a guarantee, since actual cycle length varies by person and night.
Educational estimate only. Results are not medical, legal, or financial advice. Confirm important decisions with a qualified professional.
How it works
- Choose whether you know your wake-up time or your bedtime.
- Enter that time.
- Read several candidate times, each aligned to a whole number of 90-minute sleep cycles.
Formula bedtime = wakeTime − (cycles × 90 min) − 14 min; wakeTime = bedtime + 14 min + (cycles × 90 min)
Frequently asked questions
What is a sleep cycle?
A sleep cycle is roughly 90 minutes and moves through light, deep, and REM sleep. Waking near the end of a cycle, rather than in the middle, tends to feel less groggy.
How many sleep cycles should I aim for?
Most adults do well with 5 to 6 cycles (7.5 to 9 hours). This calculator shows options from 4 to 6 cycles so you can pick what fits your schedule.
Why does the calculator add 14 minutes?
That is a typical average time it takes a healthy adult to fall asleep after getting into bed, so the cycle math starts from when you are actually asleep.
Is 90 minutes exact for everyone?
No. Actual cycle length varies by person, age, and night, roughly 70 to 120 minutes. Treat these times as a helpful estimate, not a precise schedule.
Can I use this to plan naps?
It is designed for full overnight sleep. For naps, most sleep guidance suggests either a short 20-minute nap or one full 90-minute cycle to avoid grogginess.