How the sleep calculator works
A night of sleep is made of repeating cycles of light sleep, deep sleep and REM sleep, each lasting roughly 90 minutes on average (the real length varies from about 80 to 120 minutes). Waking at the end of a cycle, when sleep is light, usually feels easier than waking from deep sleep. The calculator therefore works in whole cycles: bedtime = wake-up time − 15 minutes to fall asleep − (number of cycles × 90 minutes).
Example: what time to go to bed to wake up at 7:00 a.m.
For 5 cycles (7.5 hours of sleep): 7:00 a.m. − 7 h 30 min − 15 min = 11:15 p.m. For 6 cycles (9 hours): 9:45 p.m. For 4 cycles (6 hours): 12:45 a.m. Most adults do best with 5 or 6 cycles, so 9:45 p.m. to 11:15 p.m. is the window to aim for.
Bedtimes for common wake-up times
Wake at 5:00 a.m.: bed at 7:45 p.m. (6 cycles) or 9:15 p.m. (5 cycles). Wake at 6:00 a.m.: 8:45 p.m. or 10:15 p.m. Wake at 6:30 a.m.: 9:15 p.m. or 10:45 p.m. Wake at 7:30 a.m.: 10:15 p.m. or 11:45 p.m. Wake at 8:00 a.m.: 10:45 p.m. or 12:15 a.m. All times include 15 minutes to fall asleep.
How much sleep do you need?
The American Academy of Sleep Medicine and the CDC recommend at least 7 hours a night for adults, 8–10 hours for teenagers aged 13–18 and 9–12 hours for children aged 6–12. The recommended option in the calculator is the one closest to these adult ranges; adjust the number of cycles if you are younger or need more sleep.
Limits of the cycle method
Cycle length differs from person to person and even from night to night, and the first cycles of the night contain more deep sleep than the later ones. Treat the times as a starting point: if you regularly wake up tired, try moving your bedtime 15 minutes earlier or later for a week. Persistent insomnia, loud snoring or daytime sleepiness are worth discussing with a doctor.