Sleep Calculator
Find the best time to sleep or wake up based on 90-minute REM cycles.
Recommended Bedtimes
Accounts for the average 14 minutes it takes to fall asleep.
Waking between cycles, not during one
Sleep runs in cycles of roughly 90 minutes, moving from light sleep through deep slow-wave sleep and into REM before starting again. Waking at the end of a cycle, in light sleep, feels straightforward; waking mid-cycle out of deep sleep produces sleep inertia — the groggy, disoriented state that can persist for half an hour or more regardless of total sleep.
That is why a sleep calculator counts backwards in 90-minute blocks and adds roughly 15 minutes for falling asleep. Six full cycles is nine hours, five is seven and a half, four is six. Five complete cycles often feels better than five and a half, even though the second is more sleep, because the extra portion ends in the wrong place.
Treat the 90 minutes as an average rather than a constant. Real cycles range from about 70 to 120 minutes, vary between people, and lengthen across the night as REM takes a larger share. The calculation is a useful heuristic for choosing between two plausible alarm times, not a precise instrument — and no arrangement of cycles compensates for consistently insufficient total sleep.
Frequently asked questions
How long is one sleep cycle?
One sleep cycle lasts approximately 90 minutes and includes light sleep, deep sleep, and REM sleep stages.
How many sleep cycles do I need?
Most adults need 5–6 complete cycles (7.5–9 hours) per night for optimal rest and cognitive function.
Why does the calculator add 14 minutes?
The average person takes about 14 minutes to fall asleep after getting into bed. The 14-minute buffer accounts for this sleep onset latency.
How long is a sleep cycle?
About 90 minutes on average, cycling through light sleep, deep sleep and REM, though individual cycles range from roughly 70 to 120 minutes and lengthen across the night. Waking at the end of a cycle rather than mid-deep-sleep is what reduces grogginess.