Three ways to use it
- Wake-up time: say you need to be up at 7:00 AM. The calculator lists bedtimes that let you wake at the end of a cycle rather than in the middle of deep sleep.
- Sleep now: counts forward from the current time (or any bedtime you enter) and suggests alarm times. The clock updates by itself while the page is open.
- Hours slept: enter when you fell asleep and woke up to see your total sleep, how many cycles that was, and how it compares with the recommended range for your age.
The option closest to the middle of your age group's recommended range is marked Best.
Why 90 minutes?
Through the night you move through light sleep, deep sleep and REM in a loop that repeats four to six times, and each loop averages about 90 minutes. Waking from deep sleep leaves you groggy even after a full night; waking as a cycle ends usually feels easier. So the calculator works in multiples of 90 minutes (6, 7.5 or 9 hours) and adds the time it takes you to fall asleep, 15 minutes by default.
Examples
- Up at 7:00 AM: take away 15 minutes to fall asleep and five cycles (7.5 hours) and you should be in bed by 11:15 PM. Six cycles means 9:45 PM; four means 12:45 AM.
- In bed at 11:00 PM: you are likely asleep by 11:15, so 6:45 AM (five cycles) or 8:15 AM (six cycles) are good alarm times.
- Asleep 11:30 PM, awake 6:45 AM: 7 hours 15 minutes, about 4.8 cycles, so you woke near the end of your fifth cycle.
Recommended sleep by age
These ranges come from the National Sleep Foundation's 2015 recommendations.
| Age | Hours per day |
|---|---|
| Newborn (0–3 months) | 14–17 |
| Infant (4–11 months) | 12–15 |
| Toddler (1–2 years) | 11–14 |
| Preschool (3–5 years) | 10–13 |
| School age (6–13 years) | 9–11 |
| Teen (14–17 years) | 8–10 |
| Adult (18–64 years) | 7–9 |
| Older adult (65+) | 7–8 |
Your cycle may not be 90 minutes
Ninety minutes is an average; real cycles run roughly 70 to 120 minutes and shift from night to night. If the suggested times still leave you foggy, try setting the cycle length to 80 or 100 minutes for a few nights. For timed work sessions try the Pomodoro timer, and to add up hours across several days use the time calculator. If you are tired no matter how long you sleep, or you snore heavily, it is worth seeing a sleep specialist.