What causes elderly to lean to one side when sleeping in a reclining chair?

The chair is not reclined fully?

My mother does that, too. I can't sit in her chair because one side of it sinks. I can't sit upright. With her I think that she leans on the side where the TV and table are. Leaning requires less energy than sitting up straight. Alas, it is also bad for the muscles of the back and abdomen to sit like that. I hope don't get in that habit. (Come to think of it, I tend to lean forward like I'm about to hop up and leave. That's not good, either, I'm sure.)

Good question. I've wondered that myself but have no insight into the causes. And I've found myself doing that when I do dishes, so I'll benefit from others' answers.