Sleep Calculator (Bedtime and Wake-Up Times)

Find the bedtime or wake-up time that lines up with full 90-minute sleep cycles.

Count back in 90-minute sleep cycles from your wake-up time and add about 15 minutes to fall asleep. To wake at 7:00 AM, go to bed at 9:45 PM (6 cycles, 9 h), 11:15 PM (5 cycles, 7.5 h) or 12:45 AM (4 cycles, 6 h).

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How to use it

Choose whether you know your wake-up time or your bedtime.
Enter the time, and adjust minutes to fall asleep and cycle length if you know them.
Pick a time that gives 5 or 6 full cycles.

What you enter

Wake-up time or bedtime
Minutes to fall asleep
Cycle length

What you get

Bedtimes or wake-up times for 3–6 cycles
Hours of sleep

Formula

Bedtime = wake time − (n × cycle length) − time to fall asleep

Worked example

Input: Wake at 7:00 AM, 15 min to fall asleep, 90-min cycles

Result: Bed at 9:45 PM, 11:15 PM or 12:45 AM

11:15 PM gives 7.5 hours, meeting the 7-hour minimum.

Questions

How long is a sleep cycle?

About 90 minutes on average, ranging from roughly 70 to 120 minutes. Each cycle moves through light, deep and REM sleep.

How much sleep do adults need?

Seven or more hours a night for adults 18–60, according to the American Academy of Sleep Medicine. Teens need 8–10 hours.

Why do I feel groggy after a long sleep?

Waking in deep sleep causes sleep inertia. Waking at the end of a cycle, during lighter sleep, usually feels easier.

How long does it take to fall asleep?

Most healthy adults fall asleep in 10–20 minutes. Falling asleep in under 5 minutes can mean you are sleep deprived.

Limitations

Cycle lengths vary between people and across the night.
Not a treatment for insomnia or sleep apnoea; see a clinician for persistent problems.

References

Watson NF et al. (2015) Sleep 38(6):843-844.

More Health & Wellness

All free tools · By Dr. Zubair Khalid, DVM, MS, PhD.