Depends...
Like mal, I rarely get more than 6 hours a night. However, I am used to that, others are not. I think the most important factor, for me at least, is that if I am not getting enough sleep I feel tired during the day. When I am already tired it is just that much harder to drag myself into a workout. Bottom line. If I am too tired from lack of sleep, I won't work out efficiently, if at all.
sirant