A number of factors can contribute to hangovers:
Mild dehydration: Alcohol suppresses the release of vasopressin, a hormone produced by the brain that sends signals to the kidneys causing them to retain fluid. As a result, alcohol increases urination and excess loss of fluids. The mild dehydration that results likely contributes to hangover symptoms such as thirst, fatigue, and a headache.
Disrupted sleep: People may fall asleep faster after drinking alcohol, but their sleep is fragmented, and they tend to wake up earlier. This contributes to fatigue, as well as lost productivity.
Gastrointestinal irritation: Alcohol directly irritates the lining of the stomach and increases acid release. This can lead to nausea and stomach discomfort.
Inflammation: Alcohol increases inflammation in the body. Inflammation contributes to the malaise that people feel when they are sick, so it may play a role in hangover symptoms as well.
Acetaldehyde exposure: Alcohol metabolism, primarily by the liver, creates the compound acetaldehyde, a toxic, short-lived byproduct, which contributes to inflammation in the liver, pancreas, brain, gastrointestinal tract, and other organs.
Mini-withdrawal: While drinking, individuals may feel calmer, more relaxed, and even euphoric, but the brain quickly adjusts to those positive effects as it tries to maintain balance. As a result, when the buzz wears off, people can feel more restless and anxious than before they drank.
Because individuals are so different, it is difficult to predict how many drinks will cause a hangover. Any time people drink to intoxication, there is a chance they could have a hangover the next day.