There's a theory I've heard that it's not just about how much sleep you get, but when you wake up that affects how tired you are. You go through 2-5 sleep cycles a night, and if your alarm goes off in the middle of one, you'll wake feeling extremely tired. But if you wake with less sleep but not in the middle of a cycle, you'll be just fine. Ideally, we would all sleep in short bursts throughout our 24-hour day, but as that can't happen... we DO need a minimum of sleep for our bodies to regenerate. That varies between people, but there are studies correlating people who don't sleep very much with higher incidence of illness etc.
Check out
www.emedicine.com, it's really helpful. Might have to sign up for a free membership, though.
I actually
blogged about caffeine and its affects - check it out. You'd be surprised and appalled at how long it sticks around. (Not an advertisement, just sharing research I already did!!)
Best of luck!