Sleep Calculator
This free sleep calculator suggests bedtimes or wake up times based on 90 minute sleep cycles, so you're more likely to wake up between cycles instead of in the middle of one. Enter a wake up time to get suggested bedtimes, or a bedtime to get suggested wake up times. Nothing you enter is sent anywhere or stored.
How to Use It
- What Time Should I Go to Bed: enter the time you want to wake up, and choose a bedtime from the suggested list.
- What Time Will I Wake Up: enter the time you plan to go to bed, and see the suggested wake up times.
- Both update instantly as you change the time field.
How It Works
Sleep moves through repeating cycles of roughly 90 minutes each, and waking up near the end of a cycle, rather than in the middle of one, tends to feel more natural. This tool works backward from a wake up time, or forward from a bedtime, in whole 90 minute increments, and adds a fixed 15 minute allowance for the time it typically takes to fall asleep.
Formulas
bedtime = wake time − (cycles × 90 minutes) − 15 minutes
wake time = bedtime + 15 minutes + (cycles × 90 minutes)
A worked example: waking up at 7:00 AM, going to bed at 11:15 PM the night before covers 5 full cycles, 7.5 hours of sleep. Going to bed at 9:45 PM instead covers 6 full cycles, 9 hours of sleep. Both are marked "Recommended," since they fall within the 7 to 9 hour range commonly cited as appropriate for most adults. Shorter options, 4 or 3 cycles, are also shown, useful for a short night or a scheduled nap, but are not marked as the target.
Ninety minutes is a widely cited average sleep cycle length, not a measurement of your own sleep. Actual cycle length varies by person and by night, commonly somewhere between 70 and 120 minutes, so treat the suggested times as a starting point rather than an exact answer. Everything runs client-side in your browser. Nothing you enter is sent anywhere or stored.
Frequently Asked Questions
Why do the suggested times land on 90 minute increments?
Sleep moves through cycles of roughly 90 minutes, ending in a brief period of lighter sleep before the next cycle begins. Waking up between cycles, at the end of one of those 90 minute blocks, tends to feel more natural than waking in the middle of a cycle, when the body may be in deeper sleep. Timing a bedtime or wake time to land on a whole number of cycles is meant to reduce the chance of waking up mid cycle.
Why is 15 minutes added for falling asleep?
Most people do not fall asleep the instant they lie down. Fifteen minutes is a commonly used average for how long that takes, so the tool adds it on top of the sleep cycles themselves. If you know it typically takes you longer or shorter than that, mentally adjust the suggested time by the difference.
What does "Recommended" mean?
The 5 and 6 cycle options, 7.5 and 9 hours of sleep, are marked "Recommended" because they fall within the 7 to 9 hour range commonly cited as appropriate for most adults. The 3 and 4 cycle options are still shown since they can be useful for a short night or a scheduled nap, but they are not marked as the target.
How is this different from the Time Calculator?
The Time Calculator does general purpose clock math: the duration between two times, or adding and subtracting hours and minutes from a time. This tool answers one specific question, working backward or forward from a bedtime or wake up time in whole sleep cycles, and adds the sleep specific framing (cycle counts, hours of sleep, a recommended range) that a general purpose time calculator does not.
Is 90 minutes exactly how long my sleep cycles are?
No. Ninety minutes is a commonly cited average, but actual cycle length varies from person to person and night to night, often somewhere in the range of 70 to 120 minutes. This tool is a planning aid based on that average, not a measurement of your own sleep.
Is this medical advice?
No. This tool estimates convenient bedtimes and wake times based on an average sleep cycle length. It does not diagnose or treat any sleep condition. If you regularly have trouble falling asleep, staying asleep, or waking up feeling rested, talk to a doctor.