Aah, thanks for the anwser, must admit I was curious.
Back to the hangover: alcohol repels water from your body, that's why you need to take so many many pisses on a night of drinking, and why you sweat afterwards. This indeed also slows your metabolism, making you less energetic.
Now the thing is, your brain floats around in liquid inside your skull, in order to protect it from concussions. Alcohol drains your head-water dramatically, so the next morning you're left with a brain bumping right into the bony inside of your skull at every move, giving you a nasty headache.
The other thing is that alcohol is off course an addictive substance. Your body starts to ache when it is denied from it after a while (like while your sleeping) to express it's desire for more. That's why you can get grumpy, feeling tired and slow (backed up by the slow metabolism).
After a day or so the body recovers greatly from the addiction and you're sort of OK again. All this is off course only the case when you get really loaded, just a few beers usually don't do shit like this.
Hopefully that made the 'feeling like a zombie on sunday morning' thing a little more clear. And off course people, don't drink to much, saves you lots of crap.
Ok, next question anybody?