I find that the most important aspect of sleep for me is consistency. Oddly enough, during my best year of racing I was averaging 5 to 6 hours of sleep a night (off to bed at 2am, up at 7 to 8am).
In the last couple of years I've been averaging between 6 and 9 hours per night, and a lot of the time I feel *more* tired and groggy during the day.
So for me at least, consistency and regular sleep patterns is more important than sheer volume of sleep. I do think 6 to 7 hours a night is optimal for me, everything else equal.