Why does my 9 year old brother whine in his sleep?

My brother just turned 9, today, and I just noticed, because I'm currently sharing a room with him at our fathers, that he whines in his sleep. He whines, groans, pants, cries, and moans like he's in pain. This isn't the first time he's done it. He does it literally EVERY night and I don't know why. Does anybody have any ideas?