Reichu: Would anyone care to speculate on the importance of ceilings? We don't hear Shinji saying, "A floor I don't know" or "A wall I don't know".
Keisuke-kun: Well, when you lay down in bed, a ceiling is what you see… if you're Japanese. Most Japanese people have the uncanny ability to sleep on their back, which I find very uncomfortable. I think most people who have seen NGE have used the "unfamiliar ceiling" bit when at a strange house. I have, when I first came to Japan.
Reichu: Now that you mention it… When I was first settling into my first dorm, five hours from home, I'd lay down in that unfamiliar bed (about five feet off the ground!) and this scene (and another one later on) were the first things to come to mind.
