What causes a hangover in the first place?
Although most studies don't directly compare each of the four major symptoms of a hangover, Dr. Rohsenow suggests we can make some inferences from the results observed. Head pain and stomach distress can come about due to the inherent toxicity of the ethanol in alcohol, which can cause inflammation in the brain and stomach lining. Additionally, researchers have observed that the levels of certain cytokines (chemicals associated with inflammation) increase for several hours after an episode of heavy drinking.
By contrast, the commonly cited explanation of electrolyte depletion doesn't hold quite as much water: according to Dr. Rohsenow, electrolyte balance only plays "a minor role." That said, it certainly doesn't hurt to load up on electrolytes during a hangover, so there's no reason to cease your Gatorade chugging the morning after. Similarly, while blood sugar levels are high during intoxication and low during a hangover, there appears to be no link between blood sugar levels and the actual severity of your hangover.
Research conducted by Dr. Rohsenow herself indicates not only a correlation between sleep disturbances and hangover severity, but that alcohol increased these sleep disturbances when compared to a placebo. In other words, if you wake up a lot in the middle of the night after tying one on, you're more likely to have a bad hangover than someone who slept like a log.
Another frequently cited cause for hangovers is the buildup of acetaldehyde, which Dr. Rohsenow says is "probably not one of the chemicals that affects hangover itself" because it's metabolized by your body well before a hangover reaches its maximum severity. What's more, acetate (the chemical acetaldehyde breaks down into) generally disappears from the blood in six hours, but most hangovers tend to start 8-10 hours after drinking.
Suffice it to say, there's no magic bullet behind the cause of your hangovers: instead, they come about due to a (craft) cocktail of elements that will vary by your circumstance and body chemistry.