Sleep Calculator
Work out when to go to bed or wake up so you finish a full 90-minute sleep cycle, with time to fall asleep included and recommended hours for your age.
The interactive tool loads here when JavaScript runs.
About this toolWhat is the Sleep Calculator?
Tell the Sleep Calculator when you need to get up and it suggests bedtimes that give you three to six complete sleep cycles, counting back 90 minutes per cycle plus the time you usually take to drift off. Flip it around to find wake-up times for a given bedtime, or tap Sleep now to see when to set an alarm if you lie down this minute.
The cycle length and time to fall asleep are both editable, so if you know you wake naturally after 100-minute cycles or take half an hour to drop off, the times follow you instead of a textbook average. Enter your age and the options that give the amount of sleep recommended for your age group are highlighted, using the CDC summary of American Academy of Sleep Medicine and National Sleep Foundation guidance.
Matching your alarm to the end of a cycle is a rule of thumb that may make waking feel easier. Getting enough total hours matters more, and the page shows both.
- Three modes: wake-up time, bedtime and sleep now
- Editable cycle length and time to fall asleep
- Highlights options that meet the recommended hours for your age
- CDC, AASM and NSF sleep table by age group
- 12-hour or 24-hour clock
How to use the Sleep Calculator
- Pick a mode: I need to wake at, I'm going to bed at, or Sleep now.
- Enter the time, or let Sleep now read your device clock.
- Adjust minutes to fall asleep and cycle length if your own differ from 15 and 90.
- Add your age to highlight the options that meet the recommended hours for your age group.
- Choose one of the four suggested times and copy them if you want to set several alarms.
Worked examples
Up at 7:00
Input
Wake 7:00 AM, 15 minutes to fall asleep, 90-minute cycles
Output
Bed at 9:45 PM (6 cycles), 11:15 PM (5), 12:45 AM (4) or 2:15 AM (3)
Six cycles is 9 hours asleep and five is 7.5 hours; both meet the adult recommendation of 7 or more. Four cycles (6 hours) does not.
Bedtime at 11:00 PM
Input
Bed 11:00 PM, defaults
Output
Wake at 8:15 AM, 6:45 AM, 5:15 AM or 3:45 AM
11:00 plus 15 minutes plus 5 x 90 minutes is 6:45 AM, which gives 7.5 hours of sleep.
Longer personal cycle
Input
Wake 6:30 AM, falls asleep instantly, 100-minute cycles, 5 cycles
Output
Bed at 10:10 PM
Changing the cycle length moves every option. If you keep waking just before your alarm, your cycles may be longer or shorter than 90 minutes.
Background and insight
Sleep runs in repeating cycles of light non-REM sleep (stages N1 and N2), deep slow-wave sleep (N3) and REM sleep. In healthy adults a cycle averages about 90 minutes, a figure that goes back to the sleep laboratory work of Nathaniel Kleitman and William Dement in the 1950s and 1960s, but individual cycles range from roughly 70 to 120 minutes. Early cycles hold most of the deep sleep; later ones hold more REM. Being woken from deep sleep is linked with stronger grogginess, called sleep inertia, which is the idea behind timing an alarm for the end of a cycle.
The recommended totals are not cycle counts. For adults aged 18 to 60, the AASM and Sleep Research Society recommend 7 or more hours a night (2015 consensus). The AASM pediatric consensus (2016) gives 12 to 16 hours for infants aged 4 to 12 months including naps, 11 to 14 for ages 1 to 2, 10 to 13 for 3 to 5, 9 to 12 for 6 to 12 and 8 to 10 for teenagers 13 to 17. The National Sleep Foundation (2015) gives 14 to 17 hours for newborns, 7 to 9 for ages 61 to 64 and 7 to 8 from 65.
Most healthy adults fall asleep within about 10 to 20 minutes, which is why the default allowance is 15. Falling asleep within a minute or two of lying down every night can be a sign of sleep deprivation rather than good sleep.
Common use cases
Early starts
Work back from a flight, shift or exam to a bedtime that still gives enough sleep.
Late nights
Pick the least groggy alarm time after a late event when a full night is not possible.
Naps and night shifts
Use Sleep now before a daytime sleep to see one-cycle and multi-cycle wake times.
Children's bedtimes
Enter a child's age to see the recommended hours and choose a bedtime that fits school mornings.
Tips and limitations
- Keep the same wake-up time every day, weekends included; a steady schedule helps more than precise cycle timing.
- If you regularly need an alarm to drag yourself out of bed after 7.5 hours, try the 9-hour option for a week.
- A 20-minute nap avoids deep sleep; a 90-minute nap covers a full cycle. Naps in between are the most likely to leave you groggy.
- Caffeine has a half-life of around five hours, so an afternoon coffee can still delay sleep at night.
- Loud snoring, gasping, or daytime sleepiness despite enough hours are reasons to see a clinician about conditions such as sleep apnoea.
Frequently asked questions
What time should I go to bed if I wake up at 6 AM?
With 90-minute cycles and 15 minutes to fall asleep, aim for 8:45 PM (six cycles, 9 hours) or 10:15 PM (five cycles, 7.5 hours). 11:45 PM gives four cycles and only 6 hours, which is below the adult recommendation.
How long is one sleep cycle?
About 90 minutes on average in adults, but cycles range from about 70 to 120 minutes and vary through the night. Babies have shorter cycles of around 50 to 60 minutes.
Is it better to sleep 6 hours or 7.5 hours?
7.5 hours. Waking at the end of a cycle may feel easier, but adults need 7 or more hours for health, so four full cycles (6 hours) still leaves you short.
How many hours of sleep does a teenager need?
The American Academy of Sleep Medicine recommends 8 to 10 hours per 24 hours for ages 13 to 17. School-age children aged 6 to 12 need 9 to 12 hours.
Why do I wake up tired after 8 hours of sleep?
Possible reasons include waking in deep sleep, poor sleep quality from noise, alcohol or caffeine, an irregular schedule, or a sleep disorder such as sleep apnoea. If it happens most mornings, it is worth raising with a doctor.
How long should it take to fall asleep?
Usually 10 to 20 minutes for a healthy adult. Taking much longer most nights is a feature of insomnia; falling asleep in under five minutes every night can mean you are sleep deprived.
Is there an in-depth guide to the Sleep Calculator?
Yes. “Sleep Cycles and the 90-Minute Rule: What It Can and Cannot Do” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
Related Tools You May Like
Free, in-browser tools that pair well with this one.