How to use the Sleep Calculator
- Choose whether you know your wake-up time or your bedtime.
- Enter the hour, minutes and AM or PM.
- Read the times for 5 and 6 cycles, which are the usual targets for adults, and for 4 and 3 cycles if you are short on time.
- Set your alarm or plan your evening around the time you choose.
Formula used
One cycle is about 90 minutes. The calculator assumes about 15 minutes to fall asleep. Bedtime = wake time − (cycles × 90 min) − 15 min. Wake time = bedtime + 15 min + cycles × 90 min.
Worked example
Here is the Sleep Calculator worked through with the example values shown above, so you can check each step by hand.
| Inputs | |
|---|---|
| I want to | Wake up at a set time (find my bedtime) |
| Hour | 6 |
| Minute | 30 |
| AM or PM | AM |
| Result | |
| 5 cycles (7 h 30 min of sleep) | 10:45 PM |
| 6 cycles (9 h of sleep) | 9:15 PM |
| 4 cycles (6 h of sleep) | 12:15 AM |
| 3 cycles (4 h 30 min of sleep) | 1:45 AM |
What a sleep cycle is
During the night you move through light sleep, deep sleep and REM sleep, and then start again. One full pass takes about 90 minutes on average, though it varies between people and through the night. Waking at the end of a cycle, in light sleep, often feels better than being woken from deep sleep, which can leave you groggy.
To wake at 6:30 AM with 5 cycles (7 hours 30 minutes of sleep), you need to be asleep by 11:00 PM and, with 15 minutes to fall asleep, in bed at 10:45 PM. With 6 cycles (9 hours), go to bed at 9:15 PM. With 4 cycles (6 hours), bed at 12:15 AM.
If you know your bedtime
Going to bed at 11:00 PM and falling asleep around 11:15, the best times to wake are 5:15 AM (4 cycles), 6:45 AM (5 cycles) and 8:15 AM (6 cycles). Waking at 3:45 AM gives only 3 cycles and about 4 hours 30 minutes of sleep, which is not enough for most adults.
Set your alarm for one of these times rather than an arbitrary one. If your wake-up time is fixed, work backwards and choose the bedtime that fits whole cycles.
How much sleep do you need?
Most adults do best with about 7 to 9 hours a night, which is roughly 5 to 6 cycles. Teenagers need more, and some adults feel fine on a little less or need a little more. The best test is how you feel: if you wake without an alarm and have steady energy all day, your sleep is probably enough.
Regular sleep and wake times matter as much as the total. Going to bed and getting up at about the same time every day, even on weekends, helps your body clock.
Limits and tips
The 90-minute cycle is an average, and the times shown are a guide, not a guarantee. Stress, caffeine, alcohol, screens, noise and temperature all affect how well you sleep and how long it takes to fall asleep. If you often take much longer than 15 minutes, add that time to your plan.
This calculator gives general information and is not medical advice. If you have long-lasting trouble sleeping, snore loudly, feel unusually sleepy in the day or have other concerns, speak to a doctor.
Frequently asked questions
Is 90 minutes exact?
No. Cycle length varies between people and through the night, usually between 80 and 110 minutes. Treat the times as a guide.
How much sleep do adults need?
Most adults do best with about 7 to 9 hours. Needs vary, so judge by how you feel in the day.
Why does the calculator add 15 minutes?
It allows for the time it takes to fall asleep after you get into bed. Change your plan if you usually take longer or shorter.
Is it bad to wake in the middle of a cycle?
It is not harmful, but waking from deep sleep can make you feel groggy for a while. Waking between cycles can feel easier.
Does this work for naps?
Short naps of 10 to 20 minutes, or a full 90-minute nap, avoid waking from deep sleep. Naps in between can cause grogginess.
Can I use it for shift work?
Yes. The cycles are the same at any time of day, but sleeping in the day is often lighter, so allow extra time.
This calculator gives estimates for information only. Actual figures from a bank, employer, tax authority or doctor may differ. It is not financial, tax, legal or medical advice.