Sleep Calculator
Wake up refreshed, not groggy. Time your sleep around natural 90-minute cycles.
Common times
How the Sleep Calculator Works
Sleep happens in 90-minute cycles made up of light sleep, deep sleep, and REM. Waking up at the end of a cycle — rather than in the middle — is what makes you feel rested. This calculator adds 15 minutes to fall asleep, then counts backward (or forward) in 90-minute increments to find the best times.
5 to 6 cycles (7.5–9 hours) is the recommended range for most adults. Fewer than 4 cycles is generally insufficient for full recovery.
Frequently Asked Questions
How long is a sleep cycle?
A full sleep cycle lasts approximately 90 minutes and includes four stages: N1 (light sleep), N2 (light sleep), N3 (deep/slow-wave sleep), and REM (rapid eye movement). The proportion of deep sleep is higher in early cycles, while REM sleep dominates later cycles — which is why cutting sleep short often means less REM and more grogginess.
How many hours of sleep do adults need?
Most adults need 7–9 hours of sleep per night, which corresponds to 5–6 complete sleep cycles. Individual needs vary — some people function well on 7 hours (about 5 cycles), while others need 9 hours (6 cycles). Consistently sleeping fewer than 6 hours is associated with higher risks of health issues including cardiovascular disease, obesity, and cognitive decline.
What if I can't fall asleep in exactly 15 minutes?
The 15-minute estimate is an average. If you typically fall asleep faster (under 10 minutes), aim for the next cycle earlier. If you often take 20–30 minutes, factor that in by setting your bedtime 5–15 minutes earlier. The cycle math is more important than the exact sleep-onset time — being within one cycle is still much better than waking mid-cycle.
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