It's mostly at bed time, when I'm a little stressed out or more tired that usual. I don't have them during WBTB. In fact, successful WBTB usually end up with me feeling like I'm throw out of my bed because of increasing rocking mouvement; I really feel my dream body leaving the real one behind. The thing is that sometime it happens in a ND, even though I don't always achieve lucidity that way. My subconcious thinks it's funny I bet.