How the sleep calculator works
Sleep happens in cycles of light sleep, deep sleep and REM sleep. One full cycle lasts about 90 minutes. Waking up in the middle of a deep-sleep stage often causes grogginess (sleep inertia), while waking at the end of a cycle usually feels easier.
This calculator counts back (or forward) in 90-minute steps and adds about 15 minutes for the average time it takes to fall asleep. Most adults feel best with 5–6 complete cycles, which is 7.5–9 hours.
Recommended sleep by age
| Age group | Hours per night |
|---|---|
| Newborns (0–3 months) | 14–17 |
| Infants (4–11 months) | 12–15 |
| Toddlers (1–2 years) | 11–14 |
| Preschool (3–5 years) | 10–13 |
| School age (6–13 years) | 9–11 |
| Teenagers (14–17 years) | 8–10 |
| Adults (18–64 years) | 7–9 |
| Older adults (65+) | 7–8 |
Source: general guidance from the National Sleep Foundation. Individual needs vary.
Tips for better sleep
- Keep a consistent sleep and wake schedule, even on weekends.
- Keep your bedroom dark, quiet and cool.
- Avoid caffeine in the afternoon and heavy meals close to bedtime.
- Put screens away 30–60 minutes before bed.
- Get daylight and some exercise during the day.
Frequently asked questions
Is the sleep calculator accurate?
It gives a good estimate using the average 90-minute cycle, but real cycles vary from about 70 to 120 minutes between people and through the night.
Why does it add 15 minutes?
The average person takes around 10–20 minutes to fall asleep. Adjust your bedtime if you fall asleep faster or slower.
What if I can't get 7.5 hours?
Choose the option with 4 or 5 cycles, but regularly sleeping less than 7 hours is linked to health problems for most adults.
Is this medical advice?
No. See our disclaimer. If you have trouble sleeping, talk to a doctor.