EXACTLY like a hangover. If I'm slightly too active I will wake up the next morning feeling extremely dehydrated; headache, eyes hurt, intense nausea, very sickly, bad anxiety etc.. I'm wondering if some of this has to do with the fact that I have SIADH. A blood test showed that I have almost no anti-diuretic hormone so I am constantly dehydrated- but not to this extent. I only get the horrible hangover feeling upon overexertion. Anyone else like that?