What causes hangovers?

A hangover (veisalgia) is the sum of unpleasant physical effects following heavy consumption of alcohol. The most common reported characteristics of a hangover are feelings of extreme thirst, lethargy, nausea, sensitivity to sunlight and noise, and headache. Hypoglycemia, dehydration, and vitamin B12 deficiency are all theorized causes of hangover symptoms. Hangover symptoms may be felt for a variable period of time after alcohol consumption, and may persist for a short period of time (12 to 36 hours), or even longer (48 to 72 hours after the last alcoholic drink is consumed).