To calculate your sleep cycle, add about 15 minutes to fall asleep to your bedtime, then add 90 minutes for each cycle. In bed at 10:45 PM, you'd finish 5 cycles (7.5 hours of sleep) at 6:30 AM. Working backward is the same math in reverse: wake time minus 15 minutes minus 90 minutes per cycle.
The sleep calculator runs this formula for any time, in either direction. Below is exactly how it works, so you can check it by hand or adjust it for your own body.
Free toolSleep CalculatorBest bedtime or wake-up time, based on 90-minute sleep cycles.What is the sleep calculator formula?
The formula has two versions, depending on which time you already know:
- Wake-up time = bedtime + time to fall asleep + (number of cycles x 90 minutes)
- Bedtime = wake-up time - time to fall asleep - (number of cycles x 90 minutes)
SleepTimeCalc uses 15 minutes to fall asleep, because 10 to 20 minutes is typical for a healthy adult. If you know you usually take longer, use your own number. More on that in how long it takes to fall asleep.
For the number of cycles, most adults should aim for 5 (7.5 hours of sleep) or 6 (9 hours). Four cycles, or 6 hours, is a one-night fallback, not a plan.
How to calculate your sleep cycle wake-up time
Start from your bedtime and count forward. Here's the worked example for a 10:45 PM bedtime:
- 10:45 PM in bed
- Add 15 minutes to fall asleep: 11:00 PM asleep
- Add 5 cycles (5 x 90 = 450 minutes, or 7 h 30 min): 6:30 AM
| Cycles | Sleep | Wake-up time (in bed 10:45 PM) |
|---|---|---|
| 4 | 6 h 0 min | 5:00 AM |
| 5 | 7 h 30 min | 6:30 AM |
| 6 | 9 h 0 min | 8:00 AM |
To go the other way, a 6:30 AM alarm gives bedtimes of 10:45 PM (5 cycles), 9:15 PM (6 cycles) or 12:15 AM (4 cycles). The bedtime chart lists these for every common wake-up time.
End of sleep cycle calculator: every cycle boundary in one night
Each cycle ends 90 minutes after the last one, counted from the moment you fall asleep. If you're asleep by 11:00 PM, your estimated cycle ends are:
| Cycle | Ends around | Total sleep |
|---|---|---|
| 1 | 12:30 AM | 1 h 30 min |
| 2 | 2:00 AM | 3 h |
| 3 | 3:30 AM | 4 h 30 min |
| 4 | 5:00 AM | 6 h |
| 5 | 6:30 AM | 7 h 30 min |
| 6 | 8:00 AM | 9 h |
Brief wake-ups can happen near these boundaries, when sleep is lighter. Pick the row that lands closest to when you need to be up, or enter your times in the sleep calculator to get the list instantly.
How do you calculate your own sleep cycle length?
You can't measure it precisely at home, but you can get a rough idea. Ninety minutes is an average, not a rule. In a Swiss analysis of more than 6,000 lab-recorded cycles from 369 healthy people, the median cycle ran 96 minutes, with large differences between individuals, and the first cycle of the night was consistently shorter than later ones.
Two home methods can point you toward your own number.
Method 1: Note your natural wake-ups
On a few nights without an alarm, write down roughly when you fell asleep and when you woke up on your own, including brief middle-of-the-night wake-ups you remember.
Say you fell asleep around 11:20 PM and woke naturally at 4:35 AM. That's 5 h 15 min, or 315 minutes. Divided by 3 cycles, that's 105 minutes each. Divided by 4 cycles, it's about 79 minutes. You can't tell which from one night, which is why you need several.
Method 2: Test bedtimes 15 minutes apart
This is the more practical test. Keep the same alarm, say 7:00 AM, and change only your bedtime:
| Bedtime | Sleep before a 7:00 AM alarm | Cycles at 90 min |
|---|---|---|
| 11:30 PM | 7 h 15 min | 4.8 |
| 11:15 PM | 7 h 30 min | 5.0 |
| 11:00 PM | 7 h 45 min | 5.2 |
| 10:45 PM | 8 h 0 min | 5.3 |
Try each bedtime for 3 or 4 nights and rate how you feel on waking, 1 to 5. If 10:45 PM consistently feels best, your cycles may run closer to 96 minutes, since 5 x 96 is 8 hours.
How to adjust the formula for a longer or shorter cycle
Once you have a guess, swap it into the formula. Here's the bedtime for 5 cycles before a 7:00 AM alarm:
| Your cycle length | 5 cycles of sleep | Bedtime for 7:00 AM |
|---|---|---|
| 80 min | 6 h 40 min | 12:05 AM |
| 90 min | 7 h 30 min | 11:15 PM |
| 96 min | 8 h 0 min | 10:45 PM |
| 100 min | 8 h 20 min | 10:25 PM |
| 110 min | 9 h 10 min | 9:35 PM |
Notice the 80-minute row: 6 h 40 min is under the 7 hours adults need. If your cycles seem short, pick 6 of them rather than trimming sleep. How many you need is covered in how many sleep cycles you need.
Honest caveats about home estimates
- Your cycles aren't identical all night. Early ones are shorter and heavy on deep sleep; later ones are longer and heavier on REM.
- One night tells you little. Stress, alcohol, a late meal or a short night before all shift the pattern.
- Grogginess has other causes. Too little total sleep beats cycle timing every time. If you're short on hours, no bedtime math fixes that.
- Trackers are estimates too. Phones and wearables guess cycles from movement and heart rate, not brain waves.
If you're exhausted during the day despite a full night, or someone notices loud snoring with gasping or pauses in your breathing, see a doctor rather than tuning your cycle math.