How sleepytime works

The sleepytime calculator suggests sleep times and wake times that fall between sleep cycles, when waking is easier.

The gist:

• People take about fifteen minutes on average to fall asleep, so sleepytime builds that buffer into its suggestion. You can adjust this in settings.
• A full sleep is usually five or six complete sleep cycles, each about ninety minutes long.
• Being pulled out of the middle of a cycle tends to leave you tired and groggy. Waking between cycles, when sleep is lightest, feels gentler and more alert.
• Tell sleepytime when you need to be up, or when you're going to sleep, and it works in ninety-minute steps to land your wake time between cycles.

How the calculator picks a time

Give sleepytime a wake-up time or a time you're going to sleep, and it works in ninety-minute steps to find times that fall between cycles. It also adds about fifteen minutes for the time most people take to drift off.

If you need to be up at 7:00 AM and you're aiming for five complete cycles, sleepytime suggests sleeping at 11:15 PM: five ninety-minute cycles plus the fifteen-minute onset.

You'll see several options at once, each one cycle apart. Pick the one that fits the sleep you have.

When the numbers are approximate

Cycle length varies with age, stress, caffeine, and the day you've had, so these suggestions are averages, not a rule. For more on what's happening inside a cycle, read how sleep cycles work.