This is one of the things I am tracking in my training journal.
I did come across some recent research that suggests that 6-7 hours is optimal, anything more can be harmful to health as well.
Lack of sleep = stress = cortisol = belly fat
I know I don't get enough sleep, and it is one of those things that I am going to get sorted as I am going to have a LONG year of getting ready for comps, which is taxing enough on the immune system without adding this into the mix.