It also times naps and shows recommended sleep for every age.
How the sleep calculator works
Sleep is not one continuous state. Through the night you move through repeating cycles of light sleep, deep slow-wave sleep and REM, or dreaming, sleep. A cycle lasts about 90 minutes on average. If your alarm goes off in the middle of deep sleep, you are likely to feel heavy and slow for a while, a feeling called sleep inertia. Waking near the end of a cycle, when sleep is lighter, usually feels easier. BedtimeCalc counts back or forward in 90-minute steps from the time you give it and adds the time it takes you to fall asleep, which is 15 minutes by default.
Choosing a bedtime
For a 7:00 AM alarm, six cycles means falling asleep at 10:00 PM, so you should be in bed around 9:45 PM. Five cycles means being in bed at 11:15 PM for seven and a half hours of sleep. Most adults need seven to nine hours, so five or six cycles are marked as recommended. Four cycles, six hours, is better than waking in the middle of a fifth cycle when you are short on time, but it should not become a habit.
Naps
A short nap of 10 to 20 minutes keeps you in light sleep and improves alertness without grogginess. A 90-minute nap covers a full cycle, including REM sleep, and can help after a short night. Naps of 45 to 60 minutes often end during deep sleep. Napping late in the day can make it harder to fall asleep at night.
Sleep needs by age
Recommended sleep falls from 14–17 hours a day for newborns to 9–11 hours for school-age children, 8–10 hours for teenagers and 7–9 hours for adults. The calculator is a planning aid, not medical advice. Cycle length varies between people and through the night, so adjust the times if you regularly wake before the alarm. Everything runs in your browser.
Frequently asked questions
How long is a sleep cycle?
About 90 minutes on average, though it varies from roughly 80 to 110 minutes.
Why add 15 minutes?
Most adults take 10–20 minutes to fall asleep. Change it to match you.
Is 6 cycles always best?
Five or six cycles (7.5–9 hours) suit most adults.
Is BedtimeCalc free?
Yes, with no sign-up or ads.
Is this medical advice?
No. See a doctor about ongoing sleep problems.