Lifehacker just posted on what caffeine actually does to your brain. Which reminds me of an earlier post I did on why caffeine might be good for you;
As the studies pile up, I've become increasingly convinced that in moderation, the stuff isn't really bad for you; as to why everyone doesn't feel a 'need' for caffeine in the morning?
I believe it is addictive. If you haven't had it in your system, your system doesn't 'need' it. Simple as that. Different people also have different waking/sleeping cycles and at different ages need different amounts of sleep.