"The famous one is probably dehydration. Everyone will tell you, "Oh it’s because alcohol dehydrates you and that’s what’s causing the hangover."… [So you’re told to] alternate [between water and alcohol], or have a big glass of water before you go to bed, and some of that comes from the fact that you do get dehydrated. But, in fact, the dehydration does not seem to be what’s causing the hangover. You can fix the dehydration — and you’re still hung over.
[Also,] it’s probably not the case that it’s blood sugar that’s causing the hangover. When you drink, your blood sugar levels are affected. But by the time you’re hung over, your blood sugar levels are back to normal.
There’s that thing about mixing your drinks — drinking beer and then drinking wine, right? Again, no, you can do the study where you can have somebody drinking the same drink and getting to the same blood alcohol level and somebody drinking different drinks and getting to the same blood alcohol and they both get the same hangover, they both report the same symptoms.”