Why 90-minute cycles
A full night of sleep is a series of cycles through light, deep and REM stages, each lasting about 90 minutes on average. Waking at the end of a cycle — rather than from deep sleep in the middle of one — is why eight hours can sometimes feel worse than seven and a half.
How to use the results
Pick the suggested time closest to your target. Most adults need 5–6 full cycles (7.5–9 hours). The calculator adds ~15 minutes for falling asleep; adjust mentally if you know you drift off faster or slower.
Frequently asked questions
How many sleep cycles do I need?
Most adults do best with 5–6 cycles per night, which is 7.5–9 hours of actual sleep.
Why do I feel groggy after 8 hours?
You likely woke mid-cycle, from deep sleep. Waking at a cycle boundary — even slightly earlier — often feels more refreshing.
Is everyone's cycle exactly 90 minutes?
No, cycles range roughly 80–110 minutes and lengthen through the night; 90 minutes is a useful average, not a law.