4-6 hours works best for me. It's a perfect balance where I can get a lot of projects done, yet feel refreshed enough to take on a full work day.
When I get more than 9 hours of sleep, I find that I get headaches, very frequently, and it's very difficult to get rid of them.
That being said, I love taking naps. Naps are a gift from the goddesses. Naps will heal all of your ailments. Naps are the center of the universe. Naps, are life.
Also, ZDi is an extremely fun place to be at, I'm able to log in at all times during the day to pop in and say hi when I'm busy, or to sit down and spend a couple hours here.