Patrick Denard explains why shoulder pain gets worse at night. Lying down removes gravity's support, causes shoulders to sag, and puts direct pressure on whichever shoulder you're sleeping on. All three positions load the joint more than standing or sitting does.
So the first thing is that gravity stops helping. When you're in an upright position, your arm is coming down to the side. Simply put, when you're laying down, you have your shoulders sag more. That can be laying on your back. And then of course, if you're laying on your side, it's direct pressure. That puts increased stress on your shoulder compared to a standing or a seated position.
