Bedtime Calculator
Tell it when you need to wake up and it counts back in whole sleep cycles to suggest bedtimes, or enter your bedtime to see when to set the alarm. Each option is compared with the CDC’s recommended hours
Your Sleep Plan
Used only to compare each option with the CDC’s recommended hours. The cycle figure below is an adult one, so this tool is not meant for children.
A planning aid, not a medical device. It does not diagnose anything.
5 cycles = 7 h 30 min of sleep, after about 15 min to fall asleep. Within the CDC’s 7–8 hours.
Above the CDC’s 7–8 hours.
Above the CDC’s 7–8 hours.
Within the CDC’s 7–8 hours.
Below the CDC’s 7–8 hours.
Below the CDC’s 7–8 hours.
This is a planning aid, not medical advice. Sleep needs vary between people. If you often cannot fall asleep, wake tired after enough sleep or snore loudly, talk to a doctor.
How to use it
- Choose whether you know the time you must wake up or the time you are going to bed.
- Enter that time and pick the age group (adult or teen).
- Adjust the cycle length and the time you usually take to fall asleep if they differ from the defaults.
- Read the suggested time and the list of options. The highlighted ones fall within the CDC’s recommended hours for the age group.
Example
You must wake up at 7:00 AM, and you use the default 90-minute cycles and 15 minutes to fall asleep. Five cycles are 7 h 30 min, so the tool suggests going to bed at 11:15 PM, which is within the adult 7–8 hours. Six cycles (9 h) would mean 9:45 PM; four cycles (6 h) would mean 12:45 AM, below the range. Going to bed at 11:00 PM instead gives wake-up times of 5:15 AM (4 cycles), 6:45 AM (5 cycles) and 8:15 AM (6 cycles).
Why this tool exists and where its numbers come from
People often count backwards from the alarm without allowing for how long it takes to fall asleep, or for how many hours are recommended. The tool adds both. The NHLBI says sleep cycles repeat every 80 to 100 minutes, with usually four to six cycles per night; the Sleep Foundation gives about 10 to 20 minutes as a normal time to fall asleep for most adults; the CDC gives 7–8 hours for adults and 9–10 hours for teens and adds that individual needs vary. The 90-minute cycle and the 15-minute default are the midpoints of those ranges, chosen by the tool, and not figures stated by the sources. The sources do not say that waking at the end of a cycle is better, and the tool makes no such claim.
Sources
- CDC: How Much Sleep Do I Need?
- NHLBI (NIH): How Sleep Works, sleep phases and stages
- Sleep Foundation: Sleep Latency
For the full method, see the Methodology section just below.
Methodology
The arithmetic. For a wake-up time, the bedtime is wake-up time − (cycles × cycle length) − time to fall asleep. For a bedtime, the wake-up time is bedtime + time to fall asleep + (cycles × cycle length). Times wrap around midnight. The tool lists three to seven cycles. The comparison. Each option’s sleep time (cycles × cycle length) is compared with the CDC range for the chosen age group: 7–8 hours for adults and 9–10 hours for teens. The suggested time is the option whose sleep is closest to the middle of that range. If you change the cycle length, the options change and the suggestion can be a different number of cycles.
Where sleep math misleads
- Forgetting the time to fall asleep. Counting 7 h 30 min back from 7:00 AM gives 11:30 PM, but if you take 15 minutes to fall asleep you must be in bed at 11:15 PM.
- Treating 90 minutes as exact. The NHLBI range is 80 to 100 minutes. Over five cycles that is a spread of 100 minutes, 6 h 40 min against 8 h 20 min.
- Treating the highlighted time as a rule. The CDC itself notes that there is no magic number. Use the highlight as a reference, not a target to hit every night.
- Chasing the number when sleep is a problem. If you often cannot fall asleep or feel exhausted after enough hours, a calculator will not fix it; a doctor can.
Frequently asked questions
How does the bedtime calculator work? ▼
It counts back from your wake-up time in whole sleep cycles and also subtracts the time you take to fall asleep. Or, from your bedtime, it counts forward to give wake-up times.
How long is a sleep cycle? ▼
The NHLBI says the cycle starts over every 80 to 100 minutes, with usually four to six cycles per night. The tool uses 90 minutes by default, the middle of that range, and lets you change it.
How many hours of sleep do I need? ▼
The CDC gives 7–8 hours for adults and 9–10 hours for teens, and notes that there is no magic number because needs vary. The tool highlights the options that fall within the range for the age group you pick.
Is waking up at the end of a cycle better? ▼
The sources used here do not say so, and the tool makes no such claim. It only helps you plan a total sleep time that fits your schedule.
Can I use it for children? ▼
No. The cycle figures are adult ones and children need more sleep and have different cycles. The tool offers adult and teen options only.
Is it a medical tool? ▼
No. It is a planning aid and does not diagnose anything. If you struggle to sleep, wake exhausted or snore loudly, talk to a doctor. Nothing you enter leaves your browser.