In much the same way that what you eat on gaming nights can have an impact on the game itself, caffeine can have obvious effects on your GMing.
If you need it and don’t get it, you’re tired (that’d be me — I’m a caffeine addict all the way). If you don’t need it but have some anyway, you could get jittery. If you have too much of it, it can make you feel weird.
It’s not a terribly profound question, but I’m curious about the caffeine habits of other GMs. When you GM, do you use caffeine to keep you going?