How the sleep calculator works
During the night you move through repeating sleep cycles: light sleep, deep (slow-wave) sleep and REM sleep. One cycle averages about 90 minutes in adults. An alarm that goes off near the end of a cycle, when sleep is lighter, tends to cause less of the groggy feeling called sleep inertia than one that interrupts deep sleep.
The calculator counts back (or forward) in 90-minute steps from your wake-up time or bedtime and adds the time you usually need to fall asleep.
Worked example
You must be up at 7:00 a.m. and normally fall asleep in 15 minutes. For five cycles (7.5 hours of sleep): 7:00 − 7 h 30 min − 15 min = 11:15 p.m. For six cycles (9 hours): 7:00 − 9 h − 15 min = 9:45 p.m. If 11:15 p.m. is unrealistic, aim for the earlier time on nights before an important day rather than the 6-hour option.
How much sleep do you need?
The National Sleep Foundation's recommended ranges:
| Age | Recommended sleep per day |
|---|---|
| Children 6–13 | 9–11 hours |
| Teenagers 14–17 | 8–10 hours |
| Adults 18–64 | 7–9 hours |
| Adults 65+ | 7–8 hours |
For most adults that means five cycles (7.5 hours) or six (9 hours). Four cycles is a short night; regularly sleeping under 7 hours is linked to poorer concentration, mood and long-term health.
Limits of the 90-minute rule
- Cycles vary from roughly 70 to 120 minutes between people and across the night, so treat the times as a starting point and adjust by 10–15 minutes based on how you feel.
- A consistent wake-up time, seven days a week, matters more for feeling rested than hitting an exact cycle boundary.
- Caffeine late in the day, alcohol and screens in bed all lengthen the time it takes to fall asleep — increase that setting if they apply.
This tool gives general guidance only. Loud snoring, gasping at night, or daytime sleepiness despite enough hours in bed are reasons to see a doctor.
Frequently asked questions
What time should I go to bed if I wake up at 6 a.m.?
With 15 minutes to fall asleep, aim for 10:15 p.m. (five cycles, 7.5 hours) or 8:45 p.m. (six cycles, 9 hours). 10:15 p.m. suits most adults.
How long is a sleep cycle?
About 90 minutes on average in adults, though individual cycles range from roughly 70 to 120 minutes. Early cycles contain more deep sleep and later ones more REM sleep.
Is 6 hours of sleep enough?
For most adults, no. Health organizations recommend at least 7 hours a night for adults. A few people do well on less, but they are rare.
Why do I feel worse after 8 hours than after 7.5?
An alarm that interrupts deep sleep can leave you groggy even after a longer night. Waking near the end of a cycle often feels easier, which is the idea behind this calculator.
Last updated: October 9, 2026