Passing this request on...
I know this is a long shot, but it has been driving me absolutely nuts. I read a book in middle school (around 1997ish) and I don't remember either the author or the title.
The story was a boy meets girl from a different culture, one from a more "civilized" culture (I think the boy), and the girl from a more tribal culture. One of the key parts of it is that the girl's culture sleeps sitting up, thinking that only animals sleep sprawled out on the ground, and as the boy adapts, he learns to do that. The whole thing has a pre-medieval feel to it.
How vague can I get? I'm not going to be upset if no one can recognize it based on that description, but I figured it was worth a try!