Sleep Calculator
Waking up mid-cycle is what makes you groggy even after eight hours. This works backwards from your alarm in 90-minute cycles, so you finish a cycle instead of being torn out of one.
Your numbers
Result
- 5 cycles (7h30)
- 11:15 PM
- 4 cycles (6h)
- 12:45 AM
- Waking up at
- 7:00 AM
| Sleep cycles | Time asleep | Go to bed at |
|---|---|---|
| 6 cycles | 9 h | 9:45 PM |
| 5 cycles | 7.5 h | 11:15 PM |
| 4 cycles | 6 h | 12:45 AM |
| 3 cycles | 4.5 h | 2:15 AM |
How the calculation works
Sleep runs in cycles of roughly 90 minutes, moving from light sleep into deep sleep and REM, then back up. Waking near the top of a cycle feels easy; waking in the middle of deep sleep does not — that is sleep inertia, the fog that no amount of extra minutes fixes.
So we take your alarm, subtract whole cycles, and subtract the time you take to actually fall asleep:
bedtime = wake-up - (cycles × 90 min) - time to fall asleep
How many cycles do you need?
Most adults do well on 5 to 6 cycles — about 7.5 to 9 hours, which lines up with the 7-9 hours recommended for adults. Four cycles (6 hours) is a survivable short night, not a plan. Three is damage control.
The 90 minutes is an average, not a law
Real cycles run roughly 80 to 110 minutes and change through the night — deep sleep dominates early, REM dominates near morning. Nobody is exactly 90. Use these times as targets to aim at, and let how you actually feel on waking tell you which one is yours.
Informational and educational result. Not a substitute for professional advice.
Frequently asked questions
Sources
- National Institutes of Health (NIH) — Brain Basics: Understanding Sleep
- National Sleep Foundation — recommended sleep duration for adults (7-9 hours)
Embed this calculator
Copy the code and paste it into your site or blog. Credit with a link back to Contania is included, and the height adjusts itself.
<iframe src="https://contania.app/embed/health/sleep" title="Sleep Calculator — best bedtime for your wake-up time" width="100%" height="720" loading="lazy" style="border:1px solid #e5e0d8;border-radius:12px;max-width:640px"></iframe>
<script>window.addEventListener("message",function(e){if(e.data&&e.data.type==="contania:embed-height"){var f=document.querySelector('iframe[src="https://contania.app/embed/health/sleep"]');if(f){f.style.height=e.data.height+"px"}}});</script>Related calculators
Target Heart Rate
Find your maximum heart rate and training zones with the Karvonen method, from your age and resting heart rate.
CalculateTDEE
Calculate your TDEE (total daily energy expenditure) from your BMR and activity level, and get the daily calorie target to lose, maintain or gain weight.
CalculateWaist Circumference
Check the health risk linked to your waist size using the WHO thresholds for men and women. A simple marker of abdominal fat.
CalculateWaist-to-Hip
Calculate your waist-to-hip ratio (WHR) and see the health risk category from the WHO thresholds for men and women.
Calculate