How the sleep calculator works
Sleep isn't one long, even stretch. Through the night your brain moves through repeating cycles of light sleep, deep sleep and REM (dream) sleep. A full cycle takes about 90 minutes on average, though it varies from person to person and through the night, roughly between 70 and 120 minutes. Most adults go through four to six cycles a night.
Waking up in the middle of deep sleep is what often leaves you groggy, an effect known as sleep inertia. Waking near the end of a cycle, when sleep is lighter, tends to feel easier. This calculator counts back (or forward) in 90-minute steps from the time you give it, and adds the time it usually takes to fall asleep, so the suggested times land at the end of a cycle.
How to use it
- Choose I want to wake up at and enter your alarm time to see when to go to bed, or choose I'm going to bed at to see when to set your alarm.
- Heading to bed right now? Press Going to bed now to use the current time.
- Open Settings to change how long you usually take to fall asleep and to pick your age group.
- Pick one of the times marked Recommended. They give you enough sleep for your age as well as a cycle-friendly wake-up.
How much sleep do you need?
The US Centers for Disease Control and Prevention (CDC) recommends these amounts per day:
- Children 6–12: 9–12 hours
- Teens 13–17: 8–10 hours
- Adults 18–60: 7 or more hours
- Adults 61–64: 7–9 hours
- Adults 65+: 7–8 hours
Fewer cycles can get you through an early start once in a while, but regularly sleeping less than the recommended amount adds up. Waking at the end of a cycle helps you feel less groggy; it doesn't replace getting enough sleep overall.
Tips for falling asleep on time
- Keep the same wake-up time every day, including weekends. A steady wake time does more for your body clock than a steady bedtime.
- Get daylight in the morning and dim the lights in the hour before bed.
- Avoid caffeine in the afternoon and large meals or alcohol close to bedtime.
- Keep your bedroom cool, dark and quiet, and put your phone out of reach.
Frequently asked questions
Is every sleep cycle exactly 90 minutes?
No. Ninety minutes is an average. If you regularly wake up just before your alarm, or feel groggy at a suggested time, try the next option or adjust the minutes it takes you to fall asleep.
What about naps?
A short nap of 10 to 20 minutes usually stays in light sleep, so it's easy to wake from. A 90-minute nap covers a full cycle. Naps in between are the ones most likely to leave you groggy, and late-afternoon naps can make it harder to fall asleep at night.
Is this medical advice?
No. It's a planning tool based on average sleep cycles. If you often struggle to fall or stay asleep, snore loudly or feel tired however long you sleep, talk to a doctor.
Are my settings saved?
Your times and settings are remembered in your browser on this device so they're ready next time. Nothing is sent to us.