😴 Sleep Calculator
Time your bedtime or alarm to the end of a 90-minute sleep cycle.
Quick answer: Count back whole 90-minute sleep cycles plus about 15 minutes to fall asleep. To wake up at 7:00 AM, go to bed at 9:45 PM (6 cycles, 9 hours of sleep) or 11:15 PM (5 cycles, 7.5 hours).
Updated · Free · No sign-up · Works on any device
Sleep Calculator inputs
Result
Go to bed at
9:45 PM
6 sleep cycles · 9 h of sleep
Other bedtimes: 11:15 PM (5 cycles, 7.5 h) · 12:45 AM (4 cycles, 6 h)
All options
| Go to bed at | Sleep cycles | Sleep time | Recommendation |
|---|---|---|---|
| 9:45 PM | 6 | 9 h | Recommended for adults |
| 11:15 PM | 5 | 7.5 h | Recommended for adults |
| 12:45 AM | 4 | 6 h | Short – occasional nights only |
Includes 15 minutes to fall asleep.
How the sleep calculator works
Sleep runs in cycles of roughly 90 minutes. Waking at the end of a cycle – when sleep is light – feels easier than being jolted out of deep sleep. The calculator adds the time it takes to fall asleep and counts whole cycles forward or back.
Bedtime = wake-up time − (cycles × 90 min) − time to fall asleep
Wake-up time = bedtime + time to fall asleep + (cycles × 90 min)
Worked example
To wake at 7:00 AM with 15 minutes to fall asleep: 6 cycles (9 h) means bed at 9:45 PM; 5 cycles (7.5 h) means 11:15 PM; 4 cycles (6 h) means 12:45 AM. Going to bed at 11:00 PM instead, you would wake best at 5:15 AM, 6:45 AM or 8:15 AM.
Recommended sleep by age
| Age | Hours per night |
|---|---|
| Teenagers (14–17) | 8–10 |
| Adults (18–64) | 7–9 |
| Older adults (65+) | 7–8 |
Tips for better sleep
- Keep the same wake-up time every day, including weekends.
- Get daylight in the morning and dim lights and screens in the hour before bed.
- Avoid caffeine after early afternoon and large meals or alcohol close to bedtime.
- Keep the bedroom cool, dark and quiet.
- Cycle length varies between people – if you still wake groggy, shift by 10–15 minutes.
What happens in a sleep cycle
Each cycle moves from light sleep into deep slow-wave sleep and then into REM, when most dreaming happens. Early cycles contain more deep sleep, while later cycles have longer REM periods, so cutting the night short mostly costs REM sleep.
For general information only – not medical advice. Consult a healthcare professional.
Frequently asked questions
How long is a sleep cycle?
A full cycle through light, deep and REM sleep lasts roughly 80–110 minutes, averaging about 90 minutes. Most adults go through 4–6 cycles a night.
Why do I feel groggy when I wake up?
Waking from deep sleep in the middle of a cycle causes “sleep inertia”. Timing your alarm to the end of a cycle, when sleep is lighter, helps you wake more alert.
How much sleep do adults need?
Health organisations recommend 7–9 hours a night for adults aged 18–64 and 7–8 hours for those over 65. Teenagers need 8–10 hours.
How long does it take to fall asleep?
Healthy adults typically fall asleep in 10–20 minutes. The calculator assumes 15 minutes – adjust it under the advanced options if you usually take longer.