I read once that a REM sleep cycle is about 90 minutes or about 1.5 hours, and multiples of that do seem to work best for me. More recently, I've read that we should all be getting in the neighborhood of 9 hours a night.I sleep in 1.5 hour cycles
I normally sleep 4 1/2 hours, and feel pretty good about it, but my body's okay with anything as long as it's a multiple of 1.5 hours.
I've never gotten more than 5 hours a night regularly, not even as a child.