Why do you get a headache after waking up after a night of drinking?

I went out last night with a friend, and had a few drinks…I had a half and half with Guiness and Smithwicks…and 3 more Guinesses….so that is 80oz of beer. I didn’t take any shots of any sort, just the same beer all night.

What causes people to wake up hung over and with a headache? I mean I understand that the alcohol has something to do with it, but what causes my body to react the way it does?