Sleep calculator

How the sleep calculator works

While you sleep, your brain moves through a repeating pattern of stages: light sleep, deeper sleep, and REM sleep, the stage linked with vivid dreaming. One full trip through those stages is called a sleep cycle, and for most adults it lasts roughly 90 minutes on average. You get several cycles in a night.

Waking up near the end of a cycle, when sleep is lighter, tends to feel easier than being pulled out of deep sleep. That groggy, heavy feeling you sometimes get when an alarm cuts in is called sleep inertia. This calculator tries to time your alarm or your bedtime so you land between cycles instead of in the middle of one.

The formula is simple:

• Bedtime = wake-up time, minus the minutes it takes you to fall asleep, minus a whole number of 90 minute cycles.
• Wake-up time = bedtime, plus the minutes it takes you to fall asleep, plus a whole number of 90 minute cycles.

The calculator shows four options: 6 cycles (9 hours of sleep), 5 cycles (7 hours 30 minutes), 4 cycles (6 hours) and 3 cycles (4 hours 30 minutes). The default time to fall asleep is 15 minutes, which you can change if you know you usually drift off faster or slower.

How to read your result

The large time is the 5 cycle option, which gives you 7 hours 30 minutes of actual sleep. The tiles underneath list all four options. Adult sleep guidelines recommend 7 or more hours a night for most adults, so the 5 and 6 cycle times are marked as recommended. The 4 and 3 cycle times are there for short nights, such as an early flight, when a little sleep that ends between cycles beats a longer stretch that ends in the middle of one.

Use the first mode, "I want to wake up at", when your alarm time is fixed and you want to know when to be in bed. Use the second mode, "I'm going to bed now or at", when you are heading to bed and want to choose an alarm time. Tap "Use the time right now" to fill in the current time.

Treat every time here as an estimate. Cycles are not exactly 90 minutes for everyone, and they tend to change through the night, so think of the result as a good starting point and adjust by 10 or 15 minutes based on how you feel.

A worked example

Say you need to wake up at 7:00 AM and it usually takes you about 15 minutes to fall asleep, which are the values the calculator starts with.

1. Take away the 15 minutes to fall asleep: 7:00 AM becomes 6:45 AM. That is the moment your last cycle should end.
2. Count back 5 cycles of 90 minutes, which is 7 hours 30 minutes. 6:45 AM minus 7 hours 30 minutes is 11:15 PM.
3. Count back 6 cycles, 9 hours, and you get 9:45 PM. 4 cycles gives 12:45 AM, and 3 cycles gives 2:15 AM.

So for a 7:00 AM alarm, the best bedtimes are 9:45 PM or 11:15 PM, with 12:45 AM as a short night option. In the other mode, going to bed at 10:30 PM gives wake-up times of 7:45 AM (6 cycles), 6:15 AM (5 cycles), 4:45 AM (4 cycles) or 3:15 AM (3 cycles).

Mistakes to avoid

• Counting time in bed as sleep. If you read or scroll in bed for 30 minutes, your sleep starts later. Set the "minutes to fall asleep" field to match what really happens.
• Picking the short options every night. The 3 and 4 cycle times avoid a groggy alarm, but they do not give most adults enough total sleep. Regularly short nights add up.
• Changing your schedule every day. A steady bedtime and wake-up time, including on weekends, usually makes it easier to fall asleep and wake up on time.
• Hitting snooze. Snoozing can start a new stretch of light sleep that gets cut short again. Set the alarm for the time you will actually get up.
• Ignoring how you feel. If you wake up tired after a "perfect" number of cycles, your cycles may be a bit longer or shorter than 90 minutes. Shift your times by 10 to 15 minutes and see what works.

Common questions

How many sleep cycles do I need?

Most adults do well with 5 or 6 cycles, which is 7 hours 30 minutes to 9 hours of sleep. Guidelines for adults recommend at least 7 hours a night. Teenagers and children need more sleep than adults, so they should aim for the longer options.

Why does the calculator add 15 minutes?

Very few people fall asleep the second they lie down. About 15 minutes is a typical time to fall asleep, so it is added before the first cycle starts. If you know you fall asleep faster or slower, change the number and every time updates.

Is a sleep cycle always 90 minutes?

No. Ninety minutes is a common average, but the length of a cycle varies between people and from one part of the night to another. That is why these times are estimates. Use them as a guide and adjust based on how rested you feel.

When should I talk to a doctor about my sleep?

Talk to a doctor if you often lie awake for a long time, wake up many times a night, snore loudly or stop breathing in your sleep, or feel very sleepy during the day even after 7 or more hours in bed. Those can be signs of a sleep problem that timing alone will not fix, and many of them can be treated.