After a night of alcohol consumption, a drinker won't sleep as soundly as normal because the body is rebounding from alcohol's depressive effect on the system. When someone is drinking, alcohol inhibits glutamine, one of the body's natural stimulants. When the drinker stops drinking, the body tries to make up for lost time by producing more glutamine than it needs.
The increase in glutamine levels stimulates the brain while the drinker is trying to sleep, keeping them from reaching the deepest, most healing levels of slumber. This is a large contributor to the fatigue felt with a hangover. Severe glutamine rebound during a hangover also may be responsible for tremors, anxiety, restlessness and increased blood pressure.
Because alcohol is absorbed directly through the stomach, the cells that line the organ become irritated. Alcohol also promotes secretion of hydrochloric acid in the stomach, eventually causing the nerves to send a message to the brain that the stomach's contents are hurting the body and must be expelled through vomiting. This mechanism can actually lessen hangover symptoms in the long run because vomiting gets rid of the alcohol in the stomach and reduces the number of toxins the body has to deal with. The stomach's irritation may also be a factor in some of the other unpleasant symptoms of a hangover, such as diarrhea and lack of appetite.
So now we know why alcohol causes hangovers. In the following sections, we'll look at the science behind the most common hangover "cures."