i always sleep naked, and have been since i started high school. like others here, i feel way too constricted if i'm not nude, and i find it really hard to fall asleep. the only times i don't sleep naked are when i'm staying overnight at a friend's or if they're staying over with me, or if we're sharing a hotel room, and even then all i've got on is a really, really loose pair of shorts or lounge pants. usually the only thing that holds them up is a drawstring, and when i'm asleep, i've got it completely undone and loosened.
other than that, the only other times i don't sleep naked are when i'm either hammered or incredibly tired, and i just sort of pass out with all my clothes on.