"What actually causes hangovers? If alcohol is metabolized after a few hours, why do I still feel crummy the next day?"The main culprit behind wicked hangovers boils down to dehydration. This dehydration happens for a variety of reasons.
Since alcohol is a diuretic, those that drink will end up losing water since a diuretic is basically just something that increases the rate at which people urinate. Ever heard of "breaking the seal"? A person who has been drinking will be prone to needing a restroom more often and once they "break the seal", they'll often be needing many more return trips.
Another reason for the repeated restroom breaks is the hormone interaction that alcohol has with the body. Consuming alcohol suppresses the release of vasopressin, a hormone responsible for releasing water back into the body. The water instead heads towards the bladder and leaves the body. This leaves a person even more dehydrated and again, in need of a bathroom.
Finally, if a person chooses to drink more than a "blue zone" level of drinking they may feel the effects as the body tries to process it all. The body might respond by vomiting or developing diarrhea. Either way, not a pleasant way to end of the night, and again, leading to more toilet time and more dehydration.
All of this dehydration can lead a person to complain of nausea, headaches, dry mouth, and possibly a long list of other complaints.
If you drink and want to reduce hangovers, try increasing your water intake. Drink water throughout the day and have filling meals that include fruits and veggies. While you're out, continue to hydrate. Try alternating alcoholic and non-alcoholic drinks to regulate your hydration and reduce your time in the restroom. Give the body what it needs so it can function the next day.