A hangover describes the sum of unpleasant physiological effects following heavy consumption of alcoholic beverages. The most commonly reported characteristics of a hangover include headache, nausea, sensitivity to light and noise, lethargy, dysphoria, diarrhea and thirst, typically after the intoxicating effects of the alcohol begin to wear off. While a hangover can be experienced at any time, generally speaking a hangover is experienced the morning after a night of heavy drinking. In addition to the physical symptoms, a hangover may also induce psychological symptoms including heightened feelings of depression and anxiety.
Hypoglycemia, dehydration, acetaldehyde intoxication, glutamine rebound, and vitamin B12 deficiency are all theorized causes of hangover symptoms. Hangover symptoms may persist for several days after alcohol was last consumed. Approximately 25-30% of drinkers may be resistant to hangover symptoms. Some aspects of a hangover are viewed as symptoms of acute ethanol withdrawal, similar to the longer-duration effects of withdrawal from alcoholism, as determined by studying the increases in brain reward thresholds in rats (the amount of current required to receive to electrodes implanted in the lateral hypothalamus) following ethanol injection. Dehydration is caused by alcohol's ability to inhibit the release of anti-diuretic hormone. (Source: Wikipedia)