Scientists are not certain as to what causes hangovers, and there are several theories. Some potential causes include dehydration, the direct effect of alcohol on the brain, the withdrawal of alcohol, alcohol additives, alterations in the body's hormones, and an increased level of acetaldehyde, which is a product of alcohol metabolism.

Regarding additives, individuals who drink dark liquors, which contain by-products called congeners, tend to have more frequent and more severe hangovers. Although, research suggests that congener content does not influence next-day performance (sustained attention and reaction time), sleep, or perceived impairment.

Also, dehydration may play a role in hangovers. Dehydration occurs because alcohol inhibits the effect of a hormone called antidiuretic hormone (ADH). Normally ADH stimulates the kidneys to absorb water to avoid a dehydrated state. But by inhibiting ADH, water is not reabsorbed into our bodies. Instead, water is urinated out, at a rate that is greater than the amount of fluid being ingested. While hydration can help ease a hangover, it does not completely alleviate the symptoms.