Hungover. So said because common symptoms of a hangover can be attributed to the dehydrating effect of the alcohol in alcoholic beverages. Also supported by the fact that a large glass of water (or two) before bed after drinking can dramatically reduce hangover symptoms in the morning. Plus it sounds better to say "I'm dehydrated" than "I'm hungover."
"You look like you were out drinking last night."
"I'm dehydrated. Close that curtain and let me go back to sleep."