Sleep Calculator
Free sleep cycle calculator: the best times to go to bed or wake up in 90-minute sleep cycles, so the alarm lands between cycles instead of mid-dream.
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Sleep runs in ~90-minute cycles; waking at a cycle boundary feels better than waking mid-cycle with more total sleep. Times include ~15 minutes to fall asleep. Adults generally need 7–9 hours (5–6 cycles). General guidance, not medical advice.
This sleep calculator answers “what time should I go to bed?” the way sleep actually works: in cycles, not hours. Sleep runs in roughly 90-minute cycles through light, deep and REM stages, and waking at the end of a cycle feels dramatically better than being pulled out of the middle of one — which is why nine hours can sometimes feel worse than seven and a half.
Tell it when you need to wake up and it lists the bedtimes that fit 6, 5, 4 or 3 complete cycles, including the ~15 minutes an average adult takes to fall asleep. Or flip it around: give it your bedtime and it lists the wake-up times that land on a cycle boundary. Worked example: for a 7:00 am alarm, 9:45 pm buys six full cycles (9 hours) and 11:15 pm buys five (7.5 hours).
The 90-minute figure is an average — real cycles run about 80 to 110 minutes and lengthen through the night — so treat the times as good targets rather than physics. Adults generally need 7–9 hours; consistently needing an alarm to drag you awake is itself a signal. General guidance, not medical advice, and nothing you enter leaves your browser.
Frequently asked questions
What time should I go to bed?
Count backwards from your alarm in 90-minute cycles, plus about 15 minutes to fall asleep. For a 7:00 am alarm: 9:45 pm for six cycles or 11:15 pm for five — both beat 10:30 pm, which wakes you mid-cycle.
Why do I feel groggy after a long sleep?
Sleep inertia: an alarm that fires during deep sleep leaves you foggy regardless of total hours. Waking at a cycle boundary — even with slightly less sleep — usually feels cleaner.
Are sleep cycles really 90 minutes?
On average. Individual cycles run roughly 80–110 minutes and stretch as the night goes on, so the calculator's times are good targets, not exact science. A consistent schedule matters more than perfect arithmetic.
How much sleep do adults need?
7–9 hours for most adults — five to six full cycles. Regularly getting less builds a debt that weekend lie-ins only partly repay.