It has something to do with the sugar content in alcohol, coupled with the dehydration that alcohol visits on the system.
A glass or two of water and a couple Advils before bed do the trick for me.
Mixing libations is also not recommended. Whatever you're drinking, drink just that. Don't mix beer with wine, whiskey with rum, etc, etc.............
As an ex-bartender, I can also tell you that "true" drinkers..................they don't fool around with "candy" drinks. No "rum & cokes" or "seven & sevens" or "gin & tonics" or "Jack & cokes". Nothing exotic or mixed. True "believers" drink liquor "clean" or straight-up. Just Martini's and Scotch on the rocks, Vodka on the rocks, Brandy.............stuff like that. Even Tequila, just straight up. As a bartender, when a guy orders "a glass of Scotch", you know you got a seasoned veteran at the seat, probably a good tipper too.
In fact, my drink of choice, Brandy/rocks...............after years of testing (which I got to do for free), I settled on Brandy because of this steady customer I had, bartending days at a hotel. He was a traveling salesman and, every week, every Wednesday and Thursday for two year he'd pass through town and stay at the place...................and start drinking Brandy around 3:00 in the afternoon until he went to bed. Good drink, no hangover.