None of the balls seem to screw on very tight and I don't trust them to stay. The first time I wore one (center lower lip), I had it in for about an hour before I realized the ball had unscrewed itself and fallen off at some point. Would not recommend wearing these to sleep, that's how you lose a piercing.

More info, for anyone who feels like chancing it: I believe the balls are around 4mm, they're a little big for a monroe but just about perfect for a labret.