Introverts have limited reserves of energy and attention stored up for socializing with others and when they're used up, the aftermath can feel very much like a hangover from too much drinking.
After a few hours, I couldn't take it any more. I slipped away like a thief, skulking about the house, searching for a place where it was quiet. I came across a half-lit room and saw my future brother-in-law sitting in there, staring out the window. Knowing him to be an introvert himself, I decided this was my best option for escape and sat down across the room, wrapping my arms around my knees. I remember hoping he wouldn't think I was intruding upon his own solitude before I allowed myself to zone out, letting my thoughts drown out the raucous laughter from downstairs, breathing deeply and feeling the tension drain away. I don't know how long it was before my now-husband came looking for me, but I remember him laughing at finding the two introverts seeking refuge together.
This happened to me a couple of weeks ago. I travelled to a friend's wedding but got to town a few days early to go see a show and meet up with some other friends. By the time the wedding rolled around, I had spent time with so many people in different social groups that I left after the ceremony and didn't stay for dancing and karaoke or anything (sorry!). I didn't even get to congratulate the bride (so so sorry!!)...I was just done. After that, I mostly just holed up in my hotel room, reading, and walked around by myself, even though there were so many other things I could have been doing with so many other people. Several years ago, I would have felt weird and horrible about this, but I know myself well enough now that I just roll with it...I read so much of a book I was enjoying that the time spent can hardly be considered a loss.