Formula
Bedtime = wake time − (cycles × 90 min) − time to fall asleepWake time = bedtime + time to fall asleep + (cycles × 90 min) How it works
During the night you move through repeating cycles of light sleep, deep sleep and REM (dreaming) sleep. Each cycle lasts roughly 90 minutes on average, though it can range from about 70 to 120 minutes. Waking at the end of a cycle, when sleep is lightest, tends to feel easier than being pulled out of deep sleep.
This calculator counts back (or forward) in 90-minute steps and adds the time it typically takes you to drift off. Five or six cycles — 7.5 to 9 hours — fits the 7-plus hours that sleep experts recommend for adults. Three or four cycles are shown for nights when you are short on time, but regularly sleeping that little adds up to sleep debt.
Treat the times as a guide rather than a precise prediction: cycle length varies between people and through the night. A consistent schedule, even on weekends, does more for how rested you feel than timing alone. Ongoing trouble sleeping is worth discussing with a doctor; this is not medical advice.
Frequently asked questions
How long does it take a normal person to fall asleep?
Typically 10–20 minutes. Falling asleep in under 5 minutes every night can be a sign that you are not getting enough sleep.
What time format can I enter?
Use 24-hour time like 06:30 or 22:45, or 12-hour time with am/pm such as 6:30 am or 10:45 pm.