I was just thinking about this last night. Klaude my beardie loves to sleep with this chin against a vertical surface. It looks soooo uncomfortable, like it would hurt your neck. Also, every so often when he's sleeping with me when I watching a movie or tv he'll standup on his front legs (like he's doing a push up) and put his head up, but his eyes are closed and he's still sleeping. I was thinking that this and the weird vertical sleeping position could be a defense mechanism. Maybe predators will think they are awake and not attack if they are in those types of positions and the predator is more prone to attack if they are laying down. I don't know if this is the case but it made sense to me.