Enter the time your alarm goes off to see bedtimes, or the time you go to bed to see wake-up times. Each option is a whole number of sleep cycles, with 15 minutes added to fall asleep.
Bedtimes, longest night first
Sleep runs in cycles of light sleep, deep sleep and REM, each lasting about 90 minutes on average in adults, and a night holds four to six of them. Waking from deep sleep is what leaves people groggy for a while, a state called sleep inertia; waking near the end of a cycle, in lighter sleep, tends to feel easier.
So the calculator counts in whole cycles from the time you give, then adds the time it usually takes to fall asleep, which is 10 to 20 minutes for most adults. Six cycles are 9 hours of sleep, five are 7 h 30, four are 6 hours.
The limit of the method is the 90 minutes itself. Cycles run from roughly 70 to 120 minutes, change across the night and from one night to the next, and no calculator can know where yours fall. The times here are a reasonable starting point, which is why the cycle length can be changed.
How long is enough is a separate question. For adults, the American Academy of Sleep Medicine recommends 7 hours or more on a regular basis, and the options that reach it are marked. Teenagers need 8 to 10 hours and school-age children 9 to 12, so for them only the longest options apply.
On average. Measured in sleep labs, cycles run from about 70 to 120 minutes, the early ones hold more deep sleep and the later ones more REM, and the length differs between people. Ninety is the figure that fits most adults best, not a rule.
Because nobody is asleep the moment they lie down. Most adults take 10 to 20 minutes to fall asleep. Anyone who usually takes longer can set their own number, and every time moves with it.
No. Waking at the end of a cycle can make the first minutes of the morning easier, but it does not make up for sleep that did not happen. Over days, the total is what counts, and for adults the recommendation is 7 hours or more.
Partly. A nap of 20 to 30 minutes is meant to end before deep sleep starts, which is a different logic from counting whole cycles. For a longer nap, one full cycle of about 90 minutes is the usual choice, and the bedtime mode shows when it would end.
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