Uh, there's a title you don't see every day. I don't really have any idea how to explain it much better than that, though, which is why I've got videos. It happened three separate times, and not even always with the same sim. They all passed out on the floor in the same exact spot in the library and did some weird thing with their arms like they're swimming (but with arms clipping through their body) and their legs in some kind of almost lotus pose-like position except their ankles twisted in a way they should not be twisted.

Here are the videos, on YouTube:
Example One
Example Two
Example Three

????? This is such an odd glitch I can't even begin to figure out what could be causing it. I don't have any mods that would impact sleep at all. Also, they look like they're wearing their nightclothes instead of their day clothes and everybody is sleeping in the same spot. My game crashed shortly after I took the third video. Did I mess something up with my sleeping bag...?? I'm doing the homeless challenge and it's getting cold so my sim has slept in the library a few times.