About the Sleep Calculator
Waking up in the middle of a sleep cycle is why an alarm can leave you groggy even after a full night. This sleep calculator works with the natural rhythm of roughly 90-minute sleep cycles: tell it when you need to wake up and it suggests when to go to bed, or tell it when you are going to bed and it suggests the best alarm times. It can also count how many hours you slept and compare them with the recommended 7-9 hours.
How to use the Sleep Calculator
- Choose what to work out: a bedtime, a wake-up time, "Sleep now" or hours slept.
- Enter your time. Adjust how long you usually take to fall asleep (15 minutes by default).
- Pick one of the suggested times. "Recommended" gives 7.5 hours and "Best rest" gives 9 hours of sleep.
- Tap a time to see a chart of your sleep cycles for that night.
Features
- Bedtimes for a chosen wake-up time, and wake-up times for a chosen bedtime
- "Sleep now" mode using your current time
- Hours-slept counter with the 7-9 hour guideline
- Adjustable time to fall asleep and 12 or 24-hour clock
- Animated chart of light, deep and REM sleep across the night
What is a sleep cycle?
During the night you move through light sleep, deep sleep and REM (dreaming) sleep in cycles that last about 90 minutes, usually four to six times a night. Early cycles contain the most deep sleep, while later cycles contain more REM. Waking up at the end of a cycle, when sleep is lightest, usually feels easier than being woken from deep sleep.
How much sleep do you need?
Sleep health organisations recommend 7 to 9 hours a night for adults, 8 to 10 hours for teenagers and 9 to 12 hours for school-age children. Five full cycles (7.5 hours) or six cycles (9 hours) fit these guidelines for adults. Everyone is a little different and cycle length varies from person to person, so treat the suggested times as a helpful guide rather than an exact rule.
Frequently asked questions
What time should I go to bed to wake up at 7 AM?
For 7:00 AM, aim to fall asleep by 11:30 PM for five cycles (7.5 hours) or 10:00 PM for six cycles (9 hours). With 15 minutes to fall asleep, that means going to bed at 11:15 PM or 9:45 PM.
How long is one sleep cycle?
A sleep cycle lasts about 90 minutes on average, though it can range from roughly 80 to 120 minutes between people and across the night.
Why do I feel tired after 8 hours of sleep?
If your alarm goes off during deep sleep you can feel groggy even after a long night. Timing your alarm for the end of a cycle, and keeping a regular schedule, often helps.
How does the sleep counter work out hours slept?
Choose "Hours slept", enter when you went to bed and when you woke up. It works across midnight, subtracts the time it took to fall asleep and compares the result with the 7-9 hours adults need.
Is this sleep calculator medical advice?
No. It is a general guide based on average sleep cycles. If you regularly struggle with sleep, speak to a doctor.