Causes

Hangovers are multi-causal. Ethanol has a dehydrating effect by causing increased urine production (such substances are known as diuretics), which causes headaches, dry mouth, and lethargy. Dehydration causes the brain to shrink away from the skull slightly.This can be mitigated by drinking water after consumption of alcohol. Alcohol's effect on the stomach lining can account for nausea. Because of the increased NADH production during metabolism of ethanol by the enzymes alcohol dehydrogenase and aldehyde dehydrogenase, excess NADH can build up and slow down gluconeogenesis in the liver, thus causing hypoglycemia.
Another factor contributing to a hangover are the products from the breakdown of ethanol via liver enzymes. Ethanol is converted to acetaldehyde by the enzyme alcohol dehydrogenase, and then from acetaldehyde to acetic acid by the enzyme acetaldehyde dehydrogenase. Acetaldehyde (ethanal) is mildly toxic, contributing to hangover. These two reactions also require the conversion of NAD+ to NADH. With an excess of NADH, the lactate dehydrogenase reaction is driven to produce lactate from pyruvate (the end product of glycolysis) in order to regenerate NAD+ and sustain life. This diverts pyruvate from other pathways such as gluconeogenesis, thereby impairing the ability of the liver to supply glucose to tissues, especially the brain. Because glucose is the primary energy source of the brain, this lack of glucose contributes to hangover symptoms such as fatigue, weakness, mood disturbances, and decreased attention and concentration.
There are various nervous system effects: the removal of the depressive effects of alcohol in the brain probably account for the light and noise sensitivity.
In addition, it is thought that the presence of other alcohols (such as fusel oils), by-products of the alcoholic fermentation also called congeners, exaggerate many of the symptoms (cogeners may also be zinc or other metals added primarily to sweet liqueurs to enhance their flavor); this probably accounts for the mitigation of the effects when distilled alcohol, particularly vodka, is consumed instead.
The amount of congeners in the drink may also have an effect. Red wines have more congeners than white wines, and some people note less of a hangover with white wine. Some individuals have a strong negative reaction to red wine called red wine headache that can affect them within 15 minutes after drinking a single glass of red wine. The headache is usually accompanied by nausea and flushing.
In alcohol metabolism, one molecule of ethanol (the primary active ingredient in alcoholic beverages) produces 2 molecules of NADH, utilizing Vitamin B12 as a coenzyme. Over-consumption of ethanol may cause vitamin B12 deficiency as well.