Sleep Calculator
Best bedtimes or wake times based on 90-minute sleep cycles.
The time your alarm goes off. We work back to the best bedtimes.
Best bedtime (9 h sleep)
- Sleep at 21:46most rested
- 6 cycles · 9 h
- Sleep at 23:16
- 5 cycles · 7.5 h
- Sleep at 00:46
- 4 cycles · 6 h
- Sleep at 02:16
- 3 cycles · 4.5 h
Aim to be in bed at one of these times. Each is a whole number of ~90-minute cycles before your wake time, allowing about 14 minutes to fall asleep, so you wake between cycles feeling more rested.
How the sleep calculator works
Sleep runs in repeating cycles of roughly 90 minutes, moving from light sleep into deep sleep and REM and back again. Waking at the end of a cycle, while you are in light sleep, usually feels easier than being woken in the middle of one. This tool lines your alarm or bedtime up with the end of a whole number of cycles so you wake between them.
It works in two directions. Tell it when you want to wake up, and it counts backward to the bedtimes that give you 6, 5, 4, or 3 full cycles. Tell it when you are going to bed, and it counts forward to the matching wake times. Either way it first adds about 14 minutes for the time it takes to fall asleep, then adds or subtracts whole cycles of 90 minutes.
The formulas are simple:
- Bedtime for a wake target:
bedtime = wake − cycles × 90 − 14minutes. - Wake time for a bedtime:
wake = bedtime + 14 + cycles × 90minutes.
Everything is computed in minutes and wrapped around the 24-hour clock, so times that cross midnight come out right.
Worked example
Say you need to wake at 6:30 AM. That is 390 minutes after midnight. The calculator subtracts 14 minutes to fall asleep, then steps back one cycle at a time:
| Cycles | Sleep | Calculation | Bedtime |
|---|---|---|---|
| 6 | 9 h | 390 − 540 − 14 = −164 (wraps to 1276) | 21:16 |
| 5 | 7.5 h | 390 − 450 − 14 = −74 (wraps to 1366) | 22:46 |
| 4 | 6 h | 390 − 360 − 14 = 16 | 00:16 |
| 3 | 4.5 h | 390 − 270 − 14 = 106 | 01:46 |
So to wake at 6:30 feeling rested, the best target is to be in bed by 9:16 PM (six cycles) or 10:46 PM (five cycles). The negative minute totals simply wrap back into the previous evening, which is why a nine-hour night for a 6:30 wake-up starts the night before.
How to use it
- Choose whether you are planning around a wake-up time or a bedtime.
- Pick the time. The tool fills in the recommended times instantly.
- Aim for the six- or five-cycle option on a normal night; the shorter options are there for nights when that is not possible.
- Remember the times mark when you should be asleep or awake, not when to start your routine, so give yourself a few extra minutes to wind down.
Limitations
These times are a guide, not a guarantee. Cycle length differs between people and shifts across the night, and 14 minutes to fall asleep is an average that does not hold for everyone. Caffeine, stress, screens, alcohol, and an irregular schedule all change how you sleep far more than the exact minute of your alarm. Use the suggestions to build a consistent routine, and if you regularly struggle with sleep, speak to a doctor. This is general information, not medical advice.
Frequently asked questions
Why does the calculator add 14 minutes?
That is the average time it takes a healthy adult to fall asleep, known as sleep onset latency. The clock starts when you lie down, but your first sleep cycle does not begin until you are actually asleep, so the tool adds about 14 minutes before counting cycles. If you usually drop off faster or slower, treat the suggested times as a starting point and shift them by the difference.
What is a 90-minute sleep cycle?
During the night you move through stages of light sleep, deep sleep, and REM, then back toward light sleep, in a loop that lasts roughly 90 minutes. Waking near the end of a cycle, while you are in light sleep, tends to feel easier than being pulled out of deep sleep mid-cycle. The calculator spaces every suggested time one full cycle apart so you aim to wake between cycles.
How many cycles should I aim for?
Most adults do well on five or six cycles, which is about 7.5 to 9 hours of sleep. The tool also shows four and three cycles for nights when a full rest is not possible, but three cycles (about 4.5 hours) is a short night and not a habit to keep. Pick the latest bedtime or earliest wake time that still gives you the sleep you need.
Is 90 minutes exact for everyone?
No. Cycle length varies from person to person and across the night, commonly between 80 and 110 minutes, and it changes with age and sleep debt. The 90-minute figure is a widely used average, so the suggested times are a guide rather than a precise alarm. If you consistently wake groggy at these times, try shifting by 10 to 20 minutes to find your own rhythm.
Does the time cross midnight correctly?
Yes. All the math is done in minutes and wrapped around a 24-hour clock, so a bedtime that falls before midnight for an early-morning wake-up, or a wake time that lands the next morning, both show the correct clock time. The result is a time of day, not a date.