How the sleep calculator works
During the night your brain moves through repeating sleep cycles of light sleep, deep sleep and REM (dream) sleep. Each cycle lasts about 90 minutes on average. Waking up in the middle of deep sleep can leave you groggy — a feeling called sleep inertia — while waking at the end of a cycle usually feels easier.
This calculator counts back (or forward) in 90-minute cycles from the time you choose and adds the time it typically takes you to fall asleep:
Example
If you need to wake up at 7:00 AM and it takes you about 15 minutes to fall asleep, good bedtimes are:
- 9:45 PM — 6 cycles, 9 hours of sleep
- 11:15 PM — 5 cycles, 7.5 hours of sleep
- 12:45 AM — 4 cycles, 6 hours of sleep
How much sleep do you need?
Recommendations from the National Sleep Foundation and the American Academy of Sleep Medicine:
| Age group | Recommended sleep per day |
|---|---|
| Infants (4–12 months) | 12–16 hours (including naps) |
| Toddlers (1–2 years) | 11–14 hours (including naps) |
| Preschool (3–5 years) | 10–13 hours (including naps) |
| School age (6–12 years) | 9–12 hours |
| Teens (13–18 years) | 8–10 hours |
| Adults (18+) | 7–9 hours |
Tips for better sleep
- Keep a consistent schedule, even on weekends.
- Get daylight in the morning to anchor your body clock.
- Limit caffeine in the afternoon and avoid heavy meals or alcohol close to bedtime.
- Keep your bedroom cool, dark and quiet.
- Put screens away 30–60 minutes before bed.
Sleep cycle length varies from person to person (roughly 70–120 minutes) and changes through the night, so treat these times as a helpful guide. If you regularly feel unrested despite enough sleep, talk to a doctor.
Frequently asked questions
How long is a sleep cycle?
A sleep cycle lasts about 90 minutes on average, although it can range from roughly 70 to 120 minutes. Most adults go through four to six cycles per night.
What time should I go to bed to wake up at 6 AM?
If it takes you about 15 minutes to fall asleep, good bedtimes for a 6:00 AM alarm are 8:45 PM (6 cycles), 10:15 PM (5 cycles) or 11:45 PM (4 cycles).
Is it better to sleep 6 or 7 hours?
Most adults need 7 to 9 hours. In terms of cycles, waking after 6 hours (4 full cycles) may feel easier than waking after 7 hours mid-cycle, but getting 7.5 hours (5 cycles) is better for your health than 6.
Why do I wake up tired even after 8 hours?
You may be waking during deep sleep, have an irregular schedule, or experience poor sleep quality from noise, alcohol, stress or conditions such as sleep apnea. If tiredness persists, consult a doctor.
Does the calculator account for the time it takes to fall asleep?
Yes. The default is 15 minutes, which is typical for adults, but you can change it to match how long it usually takes you.
This calculator provides general information only and is not a substitute for professional medical advice, diagnosis or treatment. Always consult a doctor or other qualified health provider about your individual situation.
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