I swear, if I get any less than 8 hours of sleep, I feel like complete dogshit. If I get 8+, I feel great. If I get 12, I feel awesome. After 12 hours of sleep, a bunch of food, then some Jack3d on a mostly empty stomach, and I'm ready to lift for hours. I just can't function well on low sleep. Seems like it's about as important as diet.
Some people can do fine with less than 8, but I can't. Never could. I'll feel the deficit all day long. Stimulants don't even work.