Why does my leg ache when i sleep on it?

There are veins that run through your body that supply oxygen to your muscles and when these veins are suppressed or unable to supply the necessary oxygen to your legs or any place in your body, the muscles tighten up and you get a cramp, tingling or pain in that part of your body. You say that straightening your legs helps, because this permits the flow of oxygen to your muscles. Try sleeping with a small pillow between your knees, and see if that helps.