A pillow on your . Chest as you lie on your back with your hand a liitle higher off the bed than your elbow or a pillow at the same side as your cast at the side of your chest with the hand resting higher or your elbow at your side with your hand pointing up but do not put hand over your head.
On your back. Best to sleep on your back with your arm to your side with fingers pointed toward the ceiling. Wrap the cast with a pillow or blanket. This will help it stand independently at your side, and will protect your teeth if you swing your arm during your sleep. People do weird things during their sleep, so the pillow wrap is really an important safety measure.