How to use the sleep calculator
- Choose whether you know your wake-up time, your bedtime, or want to sleep right now.
- Enter the time. Adjust how long you usually take to fall asleep (the average is about 15 minutes).
- Pick one of the suggested times. Five or six full cycles (7.5–9 hours) suit most adults.
Why sleep cycles matter
During the night you pass through repeating sleep cycles of roughly 90 minutes, each moving from light sleep into deep sleep and then REM (dreaming) sleep. Waking at the end of a cycle, when sleep is light, usually feels easier than being pulled out of deep sleep by an alarm, which can cause grogginess known as sleep inertia. This calculator counts whole cycles backwards from your alarm or forwards from your bedtime, and adds the time it takes you to drift off.
Real cycles last anywhere from about 70 to 120 minutes and change through the night, so use the times as a starting point and adjust the cycle length if you regularly wake up just before the alarm. The total matters most: adults need 7–9 hours a night according to sleep medicine societies, teenagers 8–10. A consistent schedule, a dark and cool bedroom, and avoiding caffeine late in the day help more than precise timing.
The calculator runs entirely in your browser.
Is it better to sleep 6 or 7.5 hours?
7.5 hours (five cycles) for most adults. Regularly sleeping 6 hours or less is linked with poorer focus and health.
Why do I still feel tired after waking between cycles?
Total sleep, sleep quality, alcohol, stress and conditions such as sleep apnoea all play a role. Cycle timing is only one factor.
How long should a nap be?
Either 10–20 minutes, to stay in light sleep, or about 90 minutes for a full cycle.