While the causes of a hangover are still poorly understood, several factors are known to be involved including acetaldehyde accumulation, changes in the immune system and glucose metabolism, dehydration, metabolic acidosis, disturbed prostaglandin synthesis, increased cardiac output, vasodilation, sleep deprivation and malnutrition. Beverage-specific effects of additives or by-products such as congeners also play an important role. The symptoms occur typically after the intoxicating effect of the alcohol begins to wear off, generally the morning after a night of heavy drinking. (source : wikipedia)