How to use the sleep calculator
Pick a mode and enter a time. Results update instantly, with no calculate button, and your settings are remembered in this browser.
- Wake up at: enter the time you need to get up and see bedtimes that let you wake at the end of a sleep cycle, earliest first. This answers "what time should I go to bed?"
- Go to bed at: enter your bedtime to see the best times to set your alarm.
- Sleep now: shows wake-up times if you lie down right now, and keeps updating as the clock moves.
- Nap: calculates alarm times for a 20-minute power nap or a full-cycle nap, and can ring an alarm with the built-in nap timer.
- The Best badge marks the time that best fits the recommended sleep for the selected age. Tap any time to see its sleep cycles and add a calendar reminder.
How 90-minute sleep cycles work
During the night you move through light sleep, deep sleep and REM sleep in repeating sleep cycles. According to the Sleep Foundation, cycles last about 90 minutes on average and most people go through four to six per night. The first cycle is often shorter (70–100 minutes) and later ones run 90–120 minutes. Deep sleep is concentrated in the first half of the night, while REM periods grow longer toward morning.
The calculator uses time to fall asleep + cycle length × number of cycles. For example, to wake at 7:00 AM with 15 minutes to fall asleep, go to bed at 11:15 PM for five cycles (7.5 hours) or 9:45 PM for six cycles (9 hours).
The idea behind waking at the end of a cycle is that being woken from deep sleep tends to leave you groggier. That groggy feeling is called sleep inertia; it usually lasts 15 to 60 minutes and can last longer.
Keep in mind that 90 minutes is only an average. Cycle length differs between people and across the night, so no calculator can pinpoint the end of your cycles. Getting enough total sleep and keeping a consistent wake-up time matter far more than hitting an exact minute. Try the suggested times for a week, notice which mornings feel best, and fine-tune the cycle length and fall-asleep time in Settings.
How much sleep do I need? Recommendations by age
The U.S. Centers for Disease Control and Prevention (CDC) summarizes recommendations from the American Academy of Sleep Medicine (AASM), the Sleep Research Society (SRS) and the National Sleep Foundation (NSF). Adults aged 18–60 should sleep 7 or more hours per night on a regular basis. The "Recommended" and "Too short" labels in the results use these ranges.
- 0–3 months: 14–17 hours
- 4–12 months: 12–16 hours (including naps)
- 1–2 years: 11–14 hours (including naps)
- 3–5 years: 10–13 hours (including naps)
- 6–12 years: 9–12 hours
- 13–17 years: 8–10 hours
- 18–60 years: 7 or more hours
- 61–64 years: 7–9 hours
- 65 years and older: 7–8 hours
Nap calculator: 20 minutes or 90?
For adults, the Sleep Foundation suggests a nap of about 20 minutes and no longer than 30, short enough to wake before deep sleep. Longer naps make it more likely you will be woken from deep sleep, which leaves you groggier. If you have time, a nap of about one full cycle (around 90 minutes) is another option.
The Nap tab adds your fall-asleep time to give an alarm time. To protect nighttime sleep, nap at least 8 hours before bedtime, which for most people means before 3 p.m. Start the nap timer to keep the screen awake and ring a gentle alarm when it is time to get up.
Reminders: calendar file or browser notification
Tap a time and choose Add reminder to calendar to download a calendar file (.ics). On iPhone it opens straight into Calendar; on Android and computers, open the file to add it to Google Calendar, Outlook or Apple Calendar. Turn on Repeat daily to create a recurring bedtime or wake-up reminder.
A browser notification only works while this tab stays open, which is reliable on a computer. Phone browsers usually pause pages when the screen turns off, so for an alarm you cannot miss, use your phone's clock app.
Habits that help you sleep better
The CDC recommends these basic habits alongside a consistent schedule:
- Go to bed and get up at the same time every day, including weekends.
- Keep your bedroom quiet, relaxing and at a cool temperature.
- Turn off electronic devices at least 30 minutes before bedtime.
- Avoid large meals and alcohol before bedtime.
- Avoid caffeine in the afternoon or evening.
- Exercise regularly and maintain a healthy diet.
FAQ
Are sleep cycles really 90 minutes?
Ninety minutes is an average. The first cycle is often 70–100 minutes and later cycles 90–120 minutes, and it varies from person to person. Treat the results as a starting point. If you still wake up groggy, try an 80- or 100-minute cycle in Settings.
How many hours of sleep do I need?
Adults aged 18–60 should sleep 7 or more hours a night on a regular basis. Adults 61–64 need 7–9 hours and those 65 and older 7–8 hours. Teens (13–17) need 8–10 hours and school-age children (6–12) 9–12 hours.
Is 4.5 hours (three cycles) enough if I wake at the end of a cycle?
No. Timing your alarm to a cycle may make waking up feel easier, but it does not make up for too little sleep. That is why shorter options are labeled "Too short".
Why does the calculator add time to fall asleep?
You rarely fall asleep the moment you lie down. Most healthy people fall asleep within 15 to 20 minutes, so the default is 15. If it regularly takes you more than 30 minutes, review your sleep habits or talk to a doctor.
How long should a power nap be?
About 20 minutes, and no longer than 30, for most adults. Longer naps raise the chance of waking from deep sleep and feeling groggy; a full-cycle nap of around 90 minutes is an alternative if you have time. Nap before about 3 p.m.
Why didn't my browser notification fire?
Page notifications only work while this page stays open. Phones often pause pages when the screen is off, and Safari on iPhone supports notifications only for web apps added to the Home Screen. Use the calendar reminder or your clock app for anything important.
Can I use it for babies and toddlers?
You can see the recommended total sleep for their age, but infants have shorter sleep cycles (about 50 minutes) and need naps planned across the day, so 90-minute cycle times do not apply to them.