That's not to say researchers don't have theories. From dehydration to low blood sugar to an ethanol byproduct called acetaldehyde, there's no shortage of possible culprits for that next-day slump. The problem is, each of these theories has been debunked by hard research. That old belief about dehydration, for example, doesn't hold water; it turns out that, although ethanol consumption does contribute to dehydration, hangovers aren't associated with decreased electrolyte levels. Likewise, research on acetaldehyde shows it's not even present during hangovers.
The most widespread of these theories asserts hangovers are a mild form of alcohol withdrawal — essentially your body's response to the drug's absence. However, the NIH points out that not only can you can have a hangover after your very first drunk experience (whereas "withdrawal" by definition involves prolonged, repeated exposure), but hangovers also lack common withdrawal symptoms (seizures, hallucinations, etc.) and last for much shorter periods of time.
The most promising idea currently being discussed suggests that physical hangover symptoms are caused by compounds called cytokines, which are present in unusually high levels during a hangover. Scientists believe the immune system releases cytokines as an inflammatory response to ethanol, similar to how it responds to an infection. More research is needed to determine whether this is really the elusive hangover mechanism we've been searching for — or just another red herring. If cytokines are to blame, scientists could design a specific anti-inflammatory to cure or even prevent your worst morning-after symptoms.
Of course, if you have a cure that works for you — whether it's fatty foods, electrolyte-rich coconut water, or ibuprofen — keep doing what you're doing. Meanwhile, scientists will keep working to add one more inexplicable mystery to the "solved" pile, in hopes of more productive Sunday mornings everywhere. (Wired)