Sleep Calculator
This sleep calculator works out when to go to bed, or when to wake up, so your alarm goes off at the end of a sleep cycle instead of in the middle of deep sleep. Enter a wake-up time or a bedtime and it lists the best options.
Result
Go to bed at 11:15 PM
| 6 cycles · 9 h of sleep ✓ recommended | 9:45 PM |
|---|---|
| 5 cycles · 7.5 h of sleep ✓ recommended | 11:15 PM |
| 4 cycles · 6 h of sleep | 12:45 AM |
| 3 cycles · 4.5 h of sleep | 2:15 AM |
Times include 15 minutes to fall asleep. Adults need at least 7 hours of sleep (CDC). Not medical advice.
How Does the Sleep Calculator Work?
During the night your brain moves through a repeating pattern of light sleep, deep sleep and REM (dreaming) sleep. One full pass through these stages is a sleep cycle, and it lasts about 90 minutes on average. Waking up at the end of a cycle, when sleep is lighter, usually feels easier than being pulled out of deep sleep halfway through one.
The calculator counts backwards from your wake-up time, or forwards from your bedtime, in whole cycles. It also adds the time it takes you to fall asleep.
How to Calculate Your Bedtime by Hand
Working back from a wake-up time
Bedtime = wake-up time − (cycles × cycle length) − time to fall asleep
Working forward from a bedtime
Wake-up time = bedtime + time to fall asleep + (cycles × cycle length)
Worked example: up at 6:30 AM for work
You need to be up at 6:30 AM, and it usually takes you about 15 minutes to fall asleep. Using 90-minute cycles:
- 6 cycles (9 hours): 6:30 AM − 9 h − 15 min = 9:15 PM
- 5 cycles (7.5 hours): 6:30 AM − 7 h 30 min − 15 min = 10:45 PM
- 4 cycles (6 hours): 6:30 AM − 6 h − 15 min = 12:15 AM
10:45 PM is the sweet spot for most adults: it gives 7.5 hours of actual sleep, above the CDC’s 7-hour minimum, and wakes you at the end of a cycle.
Worked example: going to bed late
It’s 11:40 PM and you’re heading to bed now. Using the “When to wake up” option with the current time:
- 11:40 PM + 15 min + 7.5 h = 7:25 AM (5 cycles)
- 11:40 PM + 15 min + 6 h = 5:55 AM (4 cycles)
If you must be up at 7:00 AM, setting the alarm for 5:55 AM or 7:25 AM may feel better than 7:00, which falls in the middle of a cycle. The 4-cycle option is still a short night, though.
Tips for Better Sleep Timing
Keep the same wake-up time every day
A consistent wake-up time, weekends included, anchors your body clock more than any calculation. Pick a time you can keep, then use the calculator to choose a matching bedtime.
Adjust the cycle length to fit you
90 minutes is an average, not a rule. If you regularly wake up a few minutes before your alarm feeling alert, your cycles may be a little shorter. If you wake groggy at the suggested times, try 100 minutes. Small changes over a week or two will tell you more than any chart.
Count the time it really takes you to fall asleep
If you read or scroll in bed, the 15-minute default is probably too short. Be honest with the setting, or the suggested bedtime will leave you short of sleep.
Total sleep matters more than timing
Waking between cycles can make mornings easier, but it doesn’t replace enough sleep. Aim for options marked as recommended (7 hours or more). If you often can’t fall asleep, wake during the night, snore loudly or feel exhausted during the day, speak to a doctor. These can be signs of a sleep disorder that no calculator can fix.
Naps follow the same idea
A short nap of 20 minutes or so tends to stay in light sleep. A 90-minute nap covers a full cycle. Naps in between are the most likely to leave you groggy.
Frequently Asked Questions
What time should I go to bed if I wake up at 7 AM?
For 7.5 hours of sleep (five 90-minute cycles), be in bed at 11:15 PM, which allows about 15 minutes to fall asleep. For a full 9 hours (six cycles), go to bed at 9:45 PM. Both meet the CDC's recommendation of at least 7 hours for adults.
How long is a sleep cycle?
About 90 minutes on average, but it varies. The Sleep Foundation describes the first cycle of the night as roughly 70 to 100 minutes and later cycles as 90 to 120 minutes. Most people go through four to six cycles a night. If you know your cycles run longer or shorter, change the cycle length in the calculator.
How much sleep do adults need?
The CDC recommends at least 7 hours a night for adults. Teenagers and children need more. The calculator marks options that meet 7 hours as recommended. A sleep calculator helps you time your alarm, but it can't make up for too little total sleep.
Why do I feel groggy even after a full night's sleep?
Waking from deep sleep can leave you groggy for a while, an effect called sleep inertia. Timing your alarm for the end of a cycle may help, but grogginess can also come from irregular schedules, alcohol, caffeine late in the day or a sleep disorder. If you're regularly tired despite enough sleep, talk to a doctor.
How long does it take to fall asleep?
Most healthy adults fall asleep within about 15 to 20 minutes of lying down. The calculator uses 15 minutes by default. If you usually take longer, increase it so the suggested times stay accurate.
Is it better to sleep 6 hours or 7.5 hours?
7.5 hours is closer to what adults need and still lines up with whole cycles. Six hours (four cycles) can help you wake up feeling less groggy on a short night, but regularly sleeping under 7 hours is linked to health problems, according to the CDC.
Sources
- Sleep Foundation, Stages of Sleep — cycles average about 90 minutes; first cycle 70–100 minutes, later cycles 90–120 minutes; four to six cycles a night
- Sleep Foundation, How Long Should It Take to Fall Asleep? — most people fall asleep within 15–20 minutes
- CDC, FastStats — Sleep in Adults (adults need at least 7 hours of sleep a day)
- This calculator gives general timing estimates and is not medical advice. Cycle length varies between people and from night to night.
Last reviewed: