About the Sleep Calculator
Sleep runs in cycles of roughly 90 minutes. Waking at the end of a cycle usually feels easier than waking in deep sleep. This calculator suggests bedtimes or wake-up times and marks the options that meet the recommended hours for your age.
How to use the Sleep Calculator
- Choose whether you know your wake-up time or your bedtime.
- Set the time (or tap Now).
- Pick your age group and how long you take to fall asleep.
- Choose a time marked Recommended and keep it consistent every day.
Formula used
Bedtime = wake time − (number of cycles × 90 min) − time to fall asleep. Recommended sleep: teens 8–10 h, adults 7–9 h, older adults 7–8 h.
Frequently asked questions
How many hours of sleep do adults need?
Adults need 7 or more hours a night according to the American Academy of Sleep Medicine; 7–9 hours suits most people.
Why do I feel tired even after 8 hours?
Waking mid-cycle, irregular sleep times, late caffeine or screen use and sleep disorders like apnoea can all leave you groggy.
How long should it take to fall asleep?
Around 10–20 minutes is typical for healthy adults.
Last reviewed: November 7, 2026