How the sleep calculator works
Through the night you move through repeating cycles of light sleep, deep sleep and REM sleep. Each cycle lasts about 80 to 100 minutes, and most people go through four to six of them. Waking at the end of a cycle, when sleep is lighter, can feel easier than being pulled out of deep sleep.
The calculator counts back (or forward) in 90-minute cycles from the time you enter and adds the time it takes you to fall asleep. A normal time to fall asleep is roughly 10 to 20 minutes, so we start at 15. Change it if you know you’re faster or slower.
| Cycles | Sleep | What it means |
|---|---|---|
| 6 | 9 hours | Plenty for most adults; a good target if you’re catching up |
| 5 | 7.5 hours | Meets the adult minimum of 7 or more hours |
| 4 | 6 hours | A short night; fine occasionally, not as a habit |
| 3 | 4.5 hours | Emergency only |
What the calculator can’t know
Cycles aren’t exactly 90 minutes. They vary from person to person and night to night, and later cycles carry more REM sleep. Treat the times as a sensible starting point, then adjust by how you feel. If you wake before your alarm feeling fine, that’s your answer.
Getting enough total sleep matters more than landing on a cycle boundary. If the choice is 7 hours or 6 hours that “fits” the cycles, take the 7. See how much sleep you need by age.
A regular wake-up time, seven days a week, does more for your sleep than any single bedtime. Coffee late in the day? Check your cutoff with the caffeine calculator.