Sleep Calculator

Most adults need about five or six cycles a night. At the 90-minute average that is 7.5 or 9 hours of sleep, which sits inside the 7 to 9 hour range the National Sleep Foundation recommends for adults and meets the AASM and Sleep Research Society advice of 7 or more hours. Four cycles is 6 hours, below that guideline, and three cycles is 4.5 hours. Counting cycles is a way of timing the alarm; the guidelines are about the total amount of sleep, kept up night after night.

A sleep cycle is one pass through the stages of sleep, and a normal night strings several of them together. Each cycle moves through non-REM sleep and then REM sleep. N1 is the brief, light transition from waking. N2 is still light sleep but more settled, and it usually makes up the largest share of the night. N3 is deep, slow-wave sleep, when you are hardest to wake. REM sleep is when most vivid dreaming happens and most muscles are relaxed. The mix changes as the night goes on: the early cycles hold most of the deep sleep, and REM periods grow longer toward morning. A cycle averages about 90 minutes, but individual cycles commonly run anywhere from roughly 70 to 120 minutes.

There are several common reasons. Eight hours in bed is not eight hours asleep once the time to fall asleep and any time awake in the night are taken out. An alarm that rings during deep sleep can leave you groggy for a while after waking, a state called sleep inertia, which is the problem cycle timing tries to avoid. An irregular schedule, caffeine late in the day, and alcohol close to bedtime all make sleep lighter or more broken. Some sleep disorders, such as obstructive sleep apnea, can interrupt sleep many times a night without you remembering it. If you regularly feel unrefreshed or sleepy during the day despite enough time in bed, that is worth discussing with a doctor.

For a rested adult, falling asleep usually takes somewhere around 10 to 20 minutes, which is why this calculator starts at 15. Falling asleep within a few minutes every night can be a sign that you are not getting enough sleep. Regularly taking more than about 30 minutes is one of the patterns doctors ask about when they assess insomnia. Set the slider to your own typical figure: it shifts every bedtime and alarm time on the page by the same amount.

With 15 minutes to fall asleep and 90-minute cycles: to wake at 6:00 AM, go to bed at 8:45 PM for six cycles, 10:15 PM for five, or 11:45 PM for four. To wake at 7:00 AM, the same three options are 9:45 PM, 11:15 PM, and 12:45 AM. Five or six cycles meets the adult guideline of 7 or more hours; four cycles gives 6 hours. If you usually fall asleep faster or slower, or your cycles run shorter or longer, change the settings and the times move with them.

For most adults, not as a regular habit. The AASM and Sleep Research Society consensus is that adults should sleep 7 or more hours a night on a regular basis, and that routinely getting less is associated with poorer health, impaired performance, and more errors and accidents. A single short night is not a crisis, and people do differ, but feeling fine is not a reliable test: in studies of repeated sleep restriction, people tend to underestimate how impaired they have become. If 6 hours is all a night allows, four complete 90-minute cycles at least puts the alarm near the end of a cycle.

Two lengths work well for most people. A short nap of about 10 to 20 minutes tends to stay in lighter sleep, so you wake more easily and feel more alert soon afterwards. A nap of about one full cycle, roughly 90 minutes, allows a complete cycle and is more likely to end in lighter sleep. Naps in between, around 30 to 60 minutes, are more likely to wake you from deep sleep and leave you groggy for a while. Napping late in the afternoon or in the evening can make it harder to fall asleep at night. The nap helper on this page gives alarm times for a 20-minute nap and a one-cycle nap, including your time to fall asleep.

Teenagers need 8 to 10 hours of sleep per 24 hours, according to the AASM pediatric consensus for ages 13 to 18, and the National Sleep Foundation gives the same range for ages 14 to 17. Body clocks tend to shift later during adolescence, which makes an early bedtime feel harder even when an early start demands one. Choose Teenagers in the guideline setting and every option is tagged against 8 to 10 hours. At the 90-minute average, six cycles is 9 hours; five cycles, at 7.5 hours, falls short.

It is a sensible rule of thumb rather than a measurement. The 90-minute cycle is an average: real cycles run roughly 70 to 120 minutes, vary from person to person, and change through the night, so nobody can say which stage you will be in at a particular clock time. Falling asleep also takes a different amount of time on different nights. Use the times as starting points, adjust the cycle length or the time to fall asleep from experience, and put getting enough total sleep on a regular schedule first. This page is general information, not medical advice.

No. Every calculation runs in your browser, and nothing you enter is uploaded, saved, or logged. Going to bed now reads the time from your device clock only at the moment you press the button. Reloading the tab resets the page to its defaults.