90 on average; anywhere from about 70 to 120.
15 is typical.
Your settings and last times are saved in this browser only. Nothing is sent anywhere.
Need help drifting off? Play rain, fan or brown noise with a sleep timer on noise.gripe.
Sleep runs in cycles of light, deep and REM sleep, each about 90 minutes on average. Waking at the end of a cycle, rather than in the middle of deep sleep, tends to feel less groggy. The calculator counts back (or forward) whole cycles from the time you give it, and adds the time it usually takes you to fall asleep.
Health bodies generally advise adults to get at least 7 hours of sleep a night, so the options are marked by length: 5 or 6 cycles is 7.5 to 9 hours; 4 cycles is 6 hours. Pick the one that fits your evening, and try to keep it the same every day.
Roughly. Cycles vary between people and through the night, from about 70 to 120 minutes, and early cycles hold more deep sleep than later ones. Treat the times as a rule of thumb. If you keep waking groggy, nudge the cycle length or the time to fall asleep in Settings.
A short nap of 10 to 20 minutes is often suggested, since it usually ends before deep sleep. A nap of about one full cycle (90 minutes) is the other common choice. Naps in between are more likely to leave you groggy for a while, and late-afternoon naps can make it harder to sleep at night.
No. It's arithmetic on averages. If you regularly have trouble sleeping, snore loudly or feel tired however long you sleep, talk to a doctor.
No. It runs in your browser. Your settings and last times are kept in this browser's storage; export them, import them, or delete them in Settings. The calendar file is made on your device.