How to use the sleep calculator
- Pick I need to wake up at if you know your alarm time, or I am going to bed at if you know when the lights go out.
- Enter the Wake-up time or Bedtime. The Use current time button fills in right now, which answers "what time should I wake up if I go to sleep now".
- Set Minutes to fall asleep (15 by default) and Cycle length (90 minutes by default) if you know yours.
- Read the three times. Green cards add up to 7 or more hours of sleep and amber cards are shorter. Then set the time in the alarm clock.
How the times are worked out
Sleep repeats a loop of light sleep, deep sleep and REM sleep. The Sleep Foundation puts the average at about 90 minutes, with a shorter first cycle (70 to 100 minutes) and later cycles of 90 to 120 minutes. A typical night holds four to six cycles. The calculator counts back or forward in whole cycles:
- Bedtime = wake-up time minus cycles times cycle length, minus minutes to fall asleep.
- Wake-up time = bedtime plus minutes to fall asleep, plus cycles times cycle length.
Green marks 7 or more hours of sleep. The CDC recommends at least 7 hours for adults 18 to 60, 7 to 9 hours for ages 61 to 64, and 7 to 8 hours from 65.
Worked example
With the starting values (wake-up time 7:00 AM, 15 minutes to fall asleep, 90-minute cycles) the calculator shows 9:45 PM for 6 cycles (9 hours of sleep), 11:15 PM for 5 cycles (7 h 30 min) and 12:45 AM for 4 cycles (6 hours). The 11:15 PM line is 7:00 AM minus 7 h 30 min of sleep minus 15 minutes to fall asleep. Going the other way, a bedtime of 11:00 PM gives alarm times of 5:15 AM, 6:45 AM and 8:15 AM.
Limits and edge cases
Cycle length differs between people and between nights, so treat the times as a starting point. The idea that waking at a cycle boundary prevents grogginess has mixed support. A 2019 review of sleep inertia found studies disagree on whether the sleep stage you wake from matters, so total sleep time counts for more than exact timing.
It uses clock times only, so it does not adjust for the night daylight saving time starts or ends. For a short nap, use the countdown timer.
Frequently asked questions
What time should I go to bed if I wake up at 6 AM?
With 15 minutes to fall asleep and 90-minute cycles, aim for 8:45 PM (6 cycles), 10:15 PM (5 cycles) or 11:45 PM (4 cycles). The two earlier times give 9 and 7 h 30 min of sleep. The later one gives 6 hours, under the 7 hours the CDC recommends for most adults.
How many hours of sleep do I need?
The CDC recommends 7 or more hours a night for adults 18 to 60, 7 to 9 hours for ages 61 to 64, and 7 to 8 hours from age 65. Needs vary a little by person, but the CDC links enough sleep with lower risk of type 2 diabetes, heart disease and high blood pressure.
Is the 90-minute sleep cycle accurate?
It is an average. The Sleep Foundation reports a first cycle of 70 to 100 minutes and later cycles of 90 to 120 minutes, and your own cycles change from night to night. That is why you can change the cycle length and why the times are a guide, not a guarantee.
How long should it take to fall asleep?
Most adults fall asleep in about 10 to 20 minutes, so the calculator starts at 15. If you usually take longer, raise Minutes to fall asleep so your bedtime moves earlier. If it regularly takes more than 30 minutes, consider talking to a clinician.
Sources: CDC, About Sleep; Sleep Foundation, Stages of Sleep; Sleep Foundation, Sleep Latency; Hilditch and McHill, Sleep inertia: current insights (2019).