Sleep Calculator
Pick the time you need to wake up, or when you are going to bed, and get times that line up with complete 90-minute sleep cycles so you wake up between cycles instead of in the middle of one.
The average adult takes about 14 minutes to fall asleep, and a sleep cycle lasts roughly 90 minutes (anywhere from 80 to 110).
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How to use the Sleep Calculator
- Choose a mode: “I want to wake up at…” to get bedtimes, or “I'm going to bed at…” to get alarm times.
- Enter the time, or press Sleep now to use the current time.
- Adjust the settings if you usually take longer to fall asleep or know your own cycle length.
- Press Calculate and pick one of the highlighted times; 5 or 6 cycles give 7.5 to 9 hours of sleep.
- Copy the result or switch between 12h and 24h format.
How the sleep cycle formula works
A night of sleep is a series of cycles. Each one moves from light sleep (N1, N2) into deep slow-wave sleep (N3) and then into REM sleep, where most dreaming happens. One cycle lasts about 90 minutes on average. Waking at the end of a cycle, when sleep is lightest, usually feels easier than being pulled out of deep sleep.
The calculator uses two simple formulas:
- Bedtime = wake-up time − (cycles × cycle length) − time to fall asleep
- Wake-up time = bedtime + time to fall asleep + (cycles × cycle length)
Example: you need to be up at 6:30 AM. Five cycles is 5 × 90 = 450 minutes (7 h 30 min). Add 14 minutes to fall asleep for 464 minutes in total. 6:30 AM minus 7 h 44 min is 10:46 PM. For six cycles (9 hours) you would go to bed at 9:16 PM.
Bedtime chart by wake-up time
| Wake up at | 6 cycles (9 h) | 5 cycles (7.5 h) | 4 cycles (6 h) |
|---|---|---|---|
| 5:00 AM | 7:46 PM | 9:16 PM | 10:46 PM |
| 5:30 AM | 8:16 PM | 9:46 PM | 11:16 PM |
| 6:00 AM | 8:46 PM | 10:16 PM | 11:46 PM |
| 6:30 AM | 9:16 PM | 10:46 PM | 12:16 AM |
| 7:00 AM | 9:46 PM | 11:16 PM | 12:46 AM |
| 7:30 AM | 10:16 PM | 11:46 PM | 1:16 AM |
| 8:00 AM | 10:46 PM | 12:16 AM | 1:46 AM |
Times include 14 minutes to fall asleep and assume 90-minute cycles.
How much sleep do you need by age?
| Age group | Recommended sleep per day |
|---|---|
| Newborn (0–3 months) | 14–17 hours |
| Infant (4–11 months) | 12–15 hours |
| Toddler (1–2 years) | 11–14 hours |
| Preschool (3–5 years) | 10–13 hours |
| School age (6–13 years) | 9–11 hours |
| Teenager (14–17 years) | 8–10 hours |
| Adult (18–64 years) | 7–9 hours |
| Older adult (65+ years) | 7–8 hours |
These ranges follow the US National Sleep Foundation guidelines. A regular schedule, a dark and cool bedroom and less screen time before bed all help you fall asleep faster.
Frequently asked questions
How does the sleep calculator work?
Sleep runs in cycles of about 90 minutes, moving through light, deep and REM sleep. The calculator adds the time it takes you to fall asleep and whole cycles to your bedtime, or subtracts them from your wake-up time, so your alarm goes off at the end of a cycle when you are in light sleep.
What time should I go to bed if I wake up at 7 AM?
With the default 14 minutes to fall asleep and 90-minute cycles, aim for bed at 9:46 PM (6 cycles, 9 hours) or 11:16 PM (5 cycles, 7.5 hours). 12:46 AM gives only 6 hours.
How many sleep cycles do I need?
Most adults need 7 to 9 hours of sleep, which is 5 or 6 full cycles. Teenagers need more, about 8 to 10 hours, and older adults often do well on 7 to 8 hours.
Is every sleep cycle exactly 90 minutes?
No. Cycles usually last between 80 and 110 minutes and vary from person to person and through the night. 90 minutes is a useful average; change the cycle length if you know yours is different.
Why do I feel groggy even after a long sleep?
Waking up during deep sleep causes sleep inertia, that heavy, foggy feeling. Timing your alarm to the end of a cycle can help, but a consistent schedule and enough total sleep matter most.
Is this medical advice?
No. The calculator gives general estimates based on average sleep cycles. If you regularly struggle to sleep or feel tired during the day, talk to a doctor.