Waking up groggy usually means your alarm went off in the middle of a deep-sleep cycle. This free sleep calculator works in whole 90-minute sleep cycles — the average length of one full trip through light sleep, deep sleep and REM sleep — so your alarm lands at the end of a cycle, when waking feels easiest. Pick a wake-up time and get your best bedtimes, or pick a bedtime and get your best wake-up times. No sign-up, no data saved — everything is calculated instantly in your browser.
Most healthy adults take about 10–20 minutes to fall asleep. If you usually drop off faster or slower, move the slider — every result moves with it.
Your brain moves through repeating sleep cycles all night. Each cycle lasts about 90 minutes on average (the US National Heart, Lung, and Blood Institute puts a typical cycle at 80–100 minutes, and individual cycles range roughly 70–120 minutes). Each cycle passes through light sleep, deep sleep and REM (dreaming) sleep. Waking at the end of a cycle, in light sleep, usually feels much easier than being woken out of deep sleep — that is the whole idea behind timing your bedtime.
The calculator counts in whole 90-minute cycles and adds your fall-asleep time:
So 5 cycles is 7.5 hours of sleep, 6 cycles is 9 hours, and 4 cycles is 6 hours. The 90-minute figure is a population average, not a measurement of you — your own cycles vary from night to night. Total sleep time and a regular schedule matter more than hitting a cycle boundary exactly.
Recommended sleep per 24 hours, from the US Centers for Disease Control and Prevention (CDC), based on the American Academy of Sleep Medicine consensus:
| Age group | Recommended sleep |
|---|---|
| School-age children, 6–12 years | 9–12 hours |
| Teens, 13–17 years | 8–10 hours |
| Adults, 18–60 years | 7 or more hours |
| Adults, 61–64 years | 7–9 hours |
| Adults, 65 years and older | 7–8 hours |
For most adults, 5 cycles (7.5 hours) is the minimum that meets the CDC's "7 or more hours" guidance, and 6 cycles (9 hours) is ideal when you can get it — especially after a hard day, while recovering from sleep debt, or when you are unwell.
You need to wake up at 6:30 AM. With the default 15 minutes to fall asleep:
Going the other way: if you go to bed at 10:45 PM and take 15 minutes to fall asleep, your best wake-up times are 5:00 AM (4 cycles), 6:30 AM (5 cycles) or 8:00 AM (6 cycles).
About 90 minutes on average in adults. The National Heart, Lung, and Blood Institute puts a typical cycle at 80–100 minutes, and individual cycles range roughly 70–120 minutes and lengthen across the night, with more deep sleep early on and more REM sleep toward morning.
Most adults need 5 or 6 cycles — 7.5 or 9 hours of sleep. That matches the CDC recommendation of 7 or more hours a night for adults aged 18–60. Teens need 8–10 hours, which is roughly 5–6 cycles plus extra, and school-age children need 9–12 hours. Four cycles (6 hours) is below the recommended minimum for adults on a regular basis.
Because you do not fall asleep the instant you get into bed. Most healthy adults take about 10–20 minutes to drift off, so 15 minutes is the standard default. If you usually fall asleep in 5 minutes, or take 30, move the slider and the results will shift to match.
Usually, yes. Waking in light sleep at the end of a cycle tends to feel easier and less groggy than being woken out of deep sleep — that groggy feeling is called sleep inertia. But cycle length is only an average, so this is a planning guide, not a guarantee: getting enough total sleep on a regular schedule matters more.
Brief wake-ups between cycles are normal. If you are awake for a while, do not watch the clock — get up, do something quiet and dimly lit, and return to bed when you feel sleepy. If you regularly lie awake for more than 30 minutes, or feel exhausted despite enough time in bed, talk to a doctor.
It can help you plan the timing of a sleep window around a shift, but it cannot fix a shifted body clock on its own. Shift workers and frequent travellers should keep the same sleep window as consistently as possible, get bright light when they need to be alert, and keep the bedroom dark, cool and quiet during daytime sleep.
Sources: CDC — About Sleep (recommended sleep durations by age); American Academy of Sleep Medicine & Sleep Research Society joint consensus (adults should sleep 7 or more hours per night on a regular basis); National Heart, Lung, and Blood Institute — Sleep Phases and Stages (cycle length 80–100 minutes). Current as of November 2026.