Large parties with a bunch of strangers aren't the only form of social situation. When I was in college, I had a mostly regular group of friends that I'd hang around with and chat politics/philosophy/whatever till late in the might. Regularly to 2 AM, sometimes to 4 AM. These days, I spend most of my evenings alone and naturally go to sleep around 12:30 to 1:00 AM if left to my own devices. I'm definitely more of a night-owl and an introvert, but close proximity to friends, as I had in college, I would stay up real late.

I agree. I used to fall asleep on the couch at loud parties in college.

Not necessarily, some find it energizing. In my case (a night owl all my life) it also depends on the social group around me.