Pick when you need to get up, or when you're going to bed, and get times that line up with roughly 90-minute sleep cycles.
Sleep runs in repeating cycles of light sleep, deep sleep and REM (dreaming) sleep. A commonly used estimate is that one cycle lasts about 90 minutes, though real cycles vary between people and through the night, often somewhere around 80 to 110 minutes. Many people find that waking near the end of a cycle, when sleep is lighter, feels easier than being woken from deep sleep. This calculator uses that standard 90-minute estimate.
The math is simple. Going backward from a wake-up time: bedtime = wake-up time − (cycles × 90 minutes) − minutes to fall asleep. Going forward from a bedtime: wake-up time = bedtime + minutes to fall asleep + (cycles × 90 minutes). It shows 3, 4, 5 and 6 cycles, which is 4½, 6, 7½ and 9 hours of actual sleep. Times wrap around midnight on a 24-hour clock, so a 1:30 AM bedtime and a 10:45 PM bedtime both work.
Worked example: to wake at 7:00 AM with 15 minutes to fall asleep, five cycles (7½ hours) means lights out at 11:15 PM, and six cycles (9 hours) means 9:45 PM. As general guidance, health organizations commonly suggest that adults get 7 or more hours of sleep, and 7 to 9 hours is a typical recommendation, so the 5- and 6-cycle options are highlighted. These are estimates for planning, not a measurement of your own sleep.
Is a sleep cycle always 90 minutes?
No. 90 minutes is an average used for planning. Cycles vary from person to person and change through the night, so treat these times as a starting point and adjust by 10 to 15 minutes if you often wake up groggy.
Why add time to fall asleep?
Most people don't fall asleep the moment they lie down. Adding your usual time (15 minutes by default) means the cycles are counted from when you actually fall asleep, not from when you get into bed.
How much sleep do adults need?
As general guidance, adults are often advised to get 7 or more hours a night, with 7 to 9 hours a common recommendation. Individual needs vary with age, health and activity. Talk to a doctor if you're regularly tired despite enough sleep.
Can I use this for naps?
This tool is built for a night's sleep. Short naps of 10 to 30 minutes are usually planned differently, since the aim is to wake up before deep sleep. Try our timer for a quick nap alarm.