I wondered why the smart advice the Bed & Breakfast owner gave me in early 1990’s about always drinking a lot of water when one was drinking a lot of other things was really working. Likewise, I also wondered why these hangover headaches felt so different (and unpleasant)
Here comes the answer: a small video from National Geographics explains (amongst other things) that to clean up the blood the liver takes the water where it can find it easily: the brain. This then shrinks the brain that then "detaches" from the skull. Ouch!