Sleep Calculator

Find the best time to go to sleep for the time you need to wake up, or the best times to wake up if you go to bed now, based on 90-minute sleep cycles.

I want to
Try going to bed at

    Times include about 15 minutes to fall asleep.

    Why sleep cycles matter

    Through the night your brain moves through repeating cycles of light sleep, deep sleep and REM (dreaming) sleep. Each cycle lasts roughly 90 minutes. Waking up from deep sleep can leave you groggy, a feeling called sleep inertia, while waking near the end of a cycle often feels easier.

    How to use the sleep calculator

    1. Choose whether you know your wake-up time or your bedtime.
    2. Enter the time, or press "Use the time now".
    3. Pick one of the suggested times. Five or six cycles give 7.5 to 9 hours of sleep, which suits most adults.

    Example

    To wake at 7:00 AM, aim to fall asleep by 10:00 PM for six cycles (9 hours) or 11:30 PM for five cycles (7.5 hours). Adding 15 minutes to fall asleep gives bedtimes of 9:45 PM or 11:15 PM.

    Recommended sleep by age

    AgeHours per night
    School age (6–12)9 to 12
    Teens (13–18)8 to 10
    Adults (18–60)7 or more

    Tips for better sleep

    • Keep the same bedtime and wake time every day, including weekends.
    • Avoid caffeine in the afternoon and heavy meals close to bedtime.
    • Keep your bedroom dark, quiet and cool, and put screens away before bed.
    • Get daylight in the morning to help set your body clock.

    This tool is for general information and is not medical advice.

    Last reviewed October 2026.

    Frequently asked questions

    How does the sleep calculator work?

    Sleep runs in cycles of roughly 90 minutes. Waking at the end of a cycle, rather than in the middle of deep sleep, may help you feel less groggy. The calculator counts back (or forward) in 90-minute cycles and adds about 15 minutes to fall asleep.

    How much sleep do adults need?

    Health organizations such as the CDC recommend at least 7 hours a night for adults. Teenagers generally need 8 to 10 hours and school-age children 9 to 12. Five or six cycles (7.5 or 9 hours) fits most adults.

    Is every sleep cycle exactly 90 minutes?

    No. Cycles usually last somewhere around 70 to 120 minutes and vary from person to person and through the night. Treat the times as a helpful guide, and adjust by 15 minutes or so based on how you feel.

    What if I take longer to fall asleep?

    Go to bed a little earlier than the time shown. The calculator assumes about 15 minutes, which is typical, but stress, caffeine and screens can make it longer.

    Can a sleep calculator fix poor sleep?

    It can help with timing, but it is not a treatment. If you often feel tired despite enough time in bed, snore loudly or struggle to sleep, talk to a doctor.