I think Its your second trimester you have to avoid sleeping on your back. Really it cuts blood flow to the baby. Its best to lay on your left side with a pillow under the legs for comfort and thats the best blood flow position for the baby. Now if you roll over on your back in the middle of the night dont sweat it just lay back on your side. Hope you get some Good sleep :)
You can sleep on your back but from what I've heard its best to lay on your left side