What does this tool do?
Waking in the middle of a deep sleep stage can leave you groggy even after enough hours in bed. This calculator checks how many 90-minute cycles fit in your sleep window after allowing time to fall asleep.
The Formula
Complete cycles = floor((Minutes in bed − Fall-asleep minutes) ÷ 90)Frequently Asked Questions
Why do I wake up tired after eight hours?
Eight hours is a duration target, not a cycle plan. If your alarm lands mid-cycle, shifting bedtime by 15–20 minutes can sometimes help.
Are all sleep cycles the same length?
No. Cycles average about 90 minutes but vary through the night, with more deep sleep early and more REM sleep later. The 90-minute model is a simplification.