The short answer is, of course, consuming too much alcohol, but what is it specifically that causes our bodies to punish us so? It's not one thing, but an accumulation of several factors: a) alcohol makes you pee more, which speeds dehydration; b) alcohol consumption raises the production of stomach acids and slows the rate at which the stomach empties itself, both of which lead to nausea and vomiting; and c) drinking often causes blood vessels to dilate, which can cause headaches. Put 'em all together and whatcha got? Worst morning ever.