Hey fellas,
The drug Caffeine is a diuretic, plain and simple. It's not the most effective one, but it makes your body drop water none the less. It's got a bunch of other properties too obviously...
A diuretic is a substance (drug, herb, and I believe some carbohydrate derivatives) that makes your body lose water, i.e. you gotta pee!
Nothing in beer has this effect, so you can go longer on it than say Coke or Redbulls, etc.
Hope that's of some help.
Turbofan