What (scientifically speaking) creates hangover?

Obviously drinking... but what goes on inside your body? I realize dehydration is a big part, but what else?

Answer:
Vitamin B12 not as much as and low glucose levels within the brain are contributing factors. Both of these are related to the reality that the liver is preoccupied breaking down ethanol and thus can't perform adjectives of its other tasks as well.

The dehydration can, among other things, certainly decrease cranial pressure; this can grant a lollapalooza of a headache.

I heard a person's electrolyte level deplete.