Sleep calculator
Enter the time you need to wake up and get bedtimes that let you finish full 90-minute sleep cycles, or enter your bedtime to see the best times to set your alarm.
Defaults to your region. Switch to compare any other scale.
Result
9:45 PM / 11:15 PM
Recommended bedtime
- 6 cycles · 9 h of sleep9:45 PM
- 5 cycles · 7.5 h of sleep11:15 PM
- 4 cycles · 6 h of sleep12:45 AM
- 3 cycles · 4.5 h of sleep2:15 AM
Wake up between cycles, not in the middle of one
One sleep cycle lasts about 90 minutes. Adults need 7–9 hours, so 5 or 6 full cycles is the target; 4 cycles (6 h) is a short night. Real cycles vary from about 80 to 120 minutes, so treat the times as a guide.
About this tool
How the sleep calculator works
Sleep runs in cycles of roughly 90 minutes, moving through light sleep, deep sleep and REM. Waking at the end of a cycle usually feels easier than waking from deep sleep. The calculator counts back (or forward) whole cycles from your chosen time and adds the minutes you need to fall asleep.
How much sleep do adults need?
Most adults need 7–9 hours a night. That is 5 cycles (7.5 hours) or 6 cycles (9 hours). Teenagers need 8–10 hours, and 4 cycles (6 hours) is a short night for almost anyone.
Example
To wake up at 7:00 AM and fall asleep in 15 minutes, go to bed at 9:45 PM for 6 cycles or 11:15 PM for 5 cycles.
Treat it as a guide
Cycle length differs between people and through the night, from about 80 to 120 minutes. A consistent schedule matters more than hitting an exact minute.
Formula
Bedtime = Wake-up time − (cycles × 90 min) − time to fall asleepWake-up time = Bedtime + time to fall asleep + (cycles × 90 min)Sleep hours = cycles × 1.5
How it is calculated
- Choose whether you know your wake-up time or your bedtime.
- Enter that time.
- Adjust how long it usually takes you to fall asleep (15 minutes is typical).
- Pick one of the suggested times; 5 or 6 cycles gives 7.5–9 hours of sleep.
Questions
What time should I go to bed to wake up at 7 AM?
With 15 minutes to fall asleep, go to bed at 9:45 PM (6 cycles, 9 h) or 11:15 PM (5 cycles, 7.5 h). 12:45 AM gives only 4 cycles (6 h).
How long is a sleep cycle?
About 90 minutes on average, ranging from roughly 80 to 120 minutes. Each cycle moves through light sleep, deep sleep and REM sleep.
How many hours of sleep do I need?
Adults aged 18–64 generally need 7–9 hours, adults over 65 need 7–8, and teenagers 8–10 hours, according to the National Sleep Foundation.
Why do I feel groggy after a long sleep?
Waking in the middle of deep sleep causes sleep inertia, a groggy feeling that can last up to 30 minutes. Timing your alarm for the end of a cycle helps you avoid it.