I average 7-8 hours per night, which is basically as much as my body seems to need. I work for myself from home, so I don't have to get up at any particular time - I just sleep as much as I need.

This seems to work well for me, as I have had pretty much perfect physical and mental health for the last 10 years (nothing but the occasional cold about once a year or so), and I have high motivation, productivity, etc.