The best theory going today about what really causes hangovers is that they are an inflammatory response, like what happens when we get an infection. Hangovers are accompanied by elevated levels of molecules called cytokines, molecules used as communications signals by the immune system. One research team in Korea found elevated levels of interleukin-10, interleukin-12, and interferon gamma in their hung-over subjects. If you inject those into a healthy subject, that person will start to have all kinds of familiar-sounding symptoms, including nausea, gastrointestinal distress, headache, chills, and fatigue. Potentially even more interesting, higher-than-normal cytokine levels also mess up memory formation, which might account for ethanol-related lapses in recall as well.
Adam Rogers in Proof