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
These options count backward in 90-minute blocks and include a fixed 14-minute sleep-onset assumption. Real sleep cycles and time to fall asleep vary.
How the sleep calculator works
Sleep alternates through non-REM and REM phases. NHLBI describes cycles that generally restart every 80 to 100 minutes, with four to six cycles in a night. This calculator uses 90 minutes as a midpoint for generating clock-time options. It does not monitor sleep and cannot know when a user’s actual stages begin or end.
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 |
The model suggests 9:16 PM for six assumed cycles or 10:46 PM for five. Those are arithmetic options, not objectively best bedtimes or a guarantee of feeling rested. The negative minute totals wrap into the previous evening.
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 are clock-time scenarios, not a sleep assessment. Cycle length and time to fall asleep differ between people and across nights, and the calculator cannot detect awakenings, sleep debt, circadian timing, or a sleep disorder. Adults should generally prioritize adequate total sleep; the AASM and Sleep Research Society recommend seven or more hours regularly for healthy adults. Do not choose a shorter option merely to complete a whole number of modeled cycles. Seek medical advice for persistent insomnia, excessive sleepiness, breathing interruptions, or other concerning symptoms.
Sources and scope
- Jurisdiction
- General educational sleep-timing model for adults
- Data current to
- July 30, 2026
- Last updated
- How Sleep Works: Sleep Phases and Stages — National Heart, Lung, and Blood Institute
- Recommended Amount of Sleep for a Healthy Adult — American Academy of Sleep Medicine and Sleep Research Society
Frequently asked questions
Why does the calculator add 14 minutes?
Fourteen minutes is a fixed convenience assumption in this calculator, not a measurement or a value that applies to every sleeper. Time to fall asleep varies. Shift the displayed suggestions earlier or later if your usual sleep-onset time differs.
What is a 90-minute sleep cycle?
Sleep alternates through non-REM and REM phases. NHLBI says a cycle typically restarts every 80 to 100 minutes, and its length and stage composition change across the night. This calculator uses 90 minutes as a midpoint for simple clock arithmetic; it cannot detect your sleep stage or guarantee an easier awakening.
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.