Sleep Calculator
Find the best time to go to bed or wake up by counting 90-minute sleep cycles plus the time it takes you to fall asleep — with alternatives at 4, 5 and 6 cycles.
Sleep Calculator: with the default inputs, best time is 10:45 PM.
24-hour clock: 6 = 6 AM, 22 = 10 PM.
5 cycles ≈ 7.5 h, 6 ≈ 9 h. Adults need at least 7 hours.
- Sleep you'd get
- 7 h 30 min
- Time in bed
- 7 h 45 minSleep plus time to fall asleep.
Assumptions
- Sleep cycles are treated as uniform 90-minute blocks; real cycles vary between 70 and 120 minutes and lengthen through the night.
- Time to fall asleep defaults to 15 minutes, a normal sleep latency.
- Recommendations are for healthy adults; children and teenagers need more sleep.
| Cycles | Go to bed at | Sleep | |
|---|---|---|---|
| 3 | 1:45 AM | 4 h 30 min | under 7 h |
| 4 | 12:15 AM | 6 h | under 7 h |
| 5 | 10:45 PM | 7 h 30 min | ◀ selected |
| 6 | 9:15 PM | 9 h | |
| 7 | 7:45 PM | 10 h 30 min | over 10 h |
Each row assumes 15 minutes to fall asleep and 90-minute cycles.
How this is worked out
The formula
Sleep = cycles × cycle length (default 90 min) Bedtime = wake time − sleep − time to fall asleep Wake time = bedtime + time to fall asleep + sleep
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- I want to
- Choose one of 2 options.Wake up at a set time — find my bedtime · Go to bed at a set time — find my wake time
- Hour (0–23)
- 24-hour clock: 6 = 6 AM, 22 = 10 PM.from 0 to 23 · whole numbers only · defaults to 6
- Minute
- A number.from 0 to 59 · whole numbers only · defaults to 30
- Sleep cycles
- 5 cycles ≈ 7.5 h, 6 ≈ 9 h. Adults need at least 7 hours.from 3 to 7 · whole numbers only · defaults to 5
- Time to fall asleep(under More options)
- 10–20 minutes is normal. Under 5 suggests sleep debt; over 30 is worth looking into.from 0 to 90 · whole numbers only · defaults to 15
- Cycle length(under More options)
- 90 is the textbook average; real cycles run 70–120 minutes and lengthen through the night.from 70 to 120 · whole numbers only · defaults to 90
What you get back
- Best timemain answer
- Sleep you'd get
- Time in bed
- Sleep plus time to fall asleep.
What this assumes
- Sleep cycles are treated as uniform 90-minute blocks; real cycles vary between 70 and 120 minutes and lengthen through the night.
- Time to fall asleep defaults to 15 minutes, a normal sleep latency.
- Recommendations are for healthy adults; children and teenagers need more sleep.
About this calculator
Sleep isn't one continuous state. You cycle through light sleep, deep slow-wave sleep and REM roughly every 90 minutes, and waking at the end of a cycle — from light sleep — feels very different from being pulled out of deep sleep in the middle of one. This calculator works backwards from when you need to be up (or forwards from when you go to bed) in whole cycles, plus the time it realistically takes you to drop off.
How to use it
Pick whether you're solving for bedtime or wake time, enter the clock time, and choose how many cycles you want. Five cycles is 7.5 hours of sleep, which lands most adults inside the 7–9 hours the American Academy of Sleep Medicine and Sleep Research Society recommend. Under More options, adjust how long you take to fall asleep and the cycle length if you know yours differs.
The 90-minute caveat
Ninety minutes is an average across people and across the night. Individual cycles run anywhere from 70 to 120 minutes, and they aren't uniform: early cycles are heavier in deep sleep and shorter, later ones are REM-rich and longer. So "wake at the end of a cycle" is a rough target, not a precise one. If you always wake groggy at the calculated time, shift it 15–20 minutes either way for a week and see what changes. Consistency of wake time does more for alertness than hitting an exact cycle boundary.
Reading the table
The alternatives at 3–7 cycles show what a shorter or longer night buys you. Three cycles (4.5 hours) is an emergency option, not a plan; the table flags anything under 7 hours. The "time in bed" figure adds your sleep latency — if you set an alarm for 6:30 and need 7.5 hours of actual sleep, you need to be in bed and lights-out by 10:45, not 11:00.
Signals worth acting on
- Falling asleep in under 5 minutes usually means you're sleep-deprived, not a great sleeper.
- Taking more than 30 minutes most nights, or waking for long stretches, is worth raising with a doctor — insomnia is very treatable with CBT-I.
- Loud snoring plus daytime sleepiness suggests sleep apnoea, which no bedtime arithmetic will fix.
Sleep need varies by person and by age: teenagers need 8–10 hours, and older adults often sleep a bit less at night but still need about 7. The cycle math is the same; only the number of cycles changes.
Frequently asked questions
▸What time should I go to bed to wake up at 6:30?
For 7.5 hours of sleep (five 90-minute cycles) and 15 minutes to fall asleep, lights out at 10:45 PM. For six cycles (9 hours), 9:15 PM.
▸How long is a sleep cycle?
About 90 minutes on average, but anywhere from 70 to 120 minutes depending on the person and the point in the night. Early cycles are shorter and deeper; later ones are longer and richer in REM.
▸How much sleep do adults need?
At least 7 hours a night on a regular basis, per the American Academy of Sleep Medicine and Sleep Research Society consensus; most adults do best with 7–9. Teenagers need 8–10.
▸Is it better to sleep 6 hours or 7.5?
7.5. The idea that four complete cycles beats five interrupted ones misses the point — chronic short sleep degrades attention and health regardless of where in a cycle you wake.
▸Why do I feel groggy even after 8 hours?
Sleep inertia from waking in deep sleep, an inconsistent schedule, alcohol, or a sleep disorder such as apnoea. Try a fixed wake time for two weeks before adjusting the cycle math.
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