About the Sleep Calculator
Tezmy's Sleep Calculator helps you plan sleep around sleep cycles. Tell it when you need to wake up and it suggests bedtimes, or tell it when you are going to bed and it suggests wake-up times. Each suggestion is a whole number of 90-minute cycles (from 3 to 6) plus the time you usually need to fall asleep, so your alarm is more likely to ring at the end of a cycle instead of in the middle of deep sleep. It is a planning aid, not medical advice.
Updated October 2026
How to use the Sleep Calculator
- 1Choose “Wake up at…” or “Go to bed at…”, or press Going to bed now.
- 2Enter the time.
- 3Adjust how long you usually take to fall asleep (15 minutes by default).
- 4Pick one of the suggested times. 5 or 6 cycles (7.5 or 9 hours) are best for most adults.
How the sleep calculator works
In “Wake up at…” mode, the calculator counts back from your wake-up time: it takes away 3, 4, 5 or 6 cycles of 90 minutes, and then the time you need to fall asleep. The result is the time to be in bed with the lights off.
In “Go to bed at…” mode it counts forward: bedtime, plus the time to fall asleep, plus 3 to 6 cycles. “Going to bed now” fills in the current time on your device and shows when to set your alarm.
Which suggestion to choose
- 6 cycles (9 hours): best when you can, for example at weekends or when catching up.
- 5 cycles (7.5 hours): the everyday choice for most adults.
- 4 cycles (6 hours): a short night. Better than waking mid-cycle, but not a habit to keep.
- 3 cycles (4.5 hours): only when you really must, such as an early flight.
Tips for better sleep
- Keep the same wake-up time every day, including weekends. It sets your body clock more than bedtime does.
- Get daylight in the morning and dim lights in the evening.
- Avoid caffeine in the 6 to 8 hours before bed and heavy meals late at night.
- Put screens away 30 to 60 minutes before sleep, or at least turn the brightness down.
- Keep the bedroom dark, quiet and a little cool.
- If you often can't sleep or feel tired all day, talk to a doctor.
Naps and shift work
For a short nap, 20 minutes is usually enough to feel refreshed without falling into deep sleep. A 90-minute nap covers one full cycle. If you work shifts, use “Wake up at…” with the time you need to get up for your shift, whatever time of day that is: the calculator works across midnight in both directions.
On your phone or computer
On a phone, tap the time box to use your phone's own time picker, and use “Going to bed now” when you are already in bed. Then set your alarm to the suggested wake-up time. Sleep is one part of a healthy routine. Check your sleep against your age with the Age Calculator, track a work shift with the Hours Calculator, and pair it with the Calorie Calculator and the BMI Calculator when you are planning a weight goal.
Features
- Bedtimes for a set wake-up time
- Wake-up times for a set bedtime
- “Going to bed now” button
- Adjustable time to fall asleep
- Suggestions for 3 to 6 sleep cycles
- 12-hour or 24-hour times
Frequently asked questions
What time should I go to bed to wake up at 7 AM?
With 15 minutes to fall asleep, go to sleep at 9:45 PM for 6 cycles (9 hours) or 11:15 PM for 5 cycles (7.5 hours). For a shorter night, 12:45 AM gives 4 cycles (6 hours). Enter your own wake-up time to get times that fit you.
What is a sleep cycle?
During the night you move through stages of light sleep, deep sleep and REM (dreaming) sleep. One round through these stages is a sleep cycle, and it lasts about 90 minutes on average for adults, though it can be anywhere from about 70 to 120 minutes.
Why do I feel tired even after a long sleep?
Waking up during deep sleep can leave you groggy for a while, which is called sleep inertia. Waking at the end of a cycle, when sleep is lighter, often feels easier. Poor sleep quality, an irregular schedule or a sleep disorder can also leave you tired, so talk to a doctor if it happens often.
How much sleep do adults need?
Most adults need at least 7 hours a night, and many feel best with 7 to 9 hours. That is about 5 to 6 sleep cycles. Teenagers usually need 8 to 10 hours and children more.
Why does the calculator add 15 minutes?
Most people take 10 to 20 minutes to fall asleep, so the calculator adds 15 minutes before the first cycle starts. If you usually fall asleep faster or slower, change the number and the times update.
Is 6 hours of sleep enough?
Six hours is 4 full cycles, so waking after 6 hours can feel better than after 6.5. But for most adults it is less than the recommended 7 hours, and a regular lack of sleep adds up. Use short nights only now and then.
Is this calculator accurate for everyone?
It uses the average 90-minute cycle, so it is a good starting point, not an exact measurement. Your own cycles may be a bit shorter or longer. Try the suggested times for a week and move them by 10 to 15 minutes if you still wake up groggy.