You definately need to train yourself out of it. You are obviously sleeping with your arms up for most if not all of the night. If you were moving regularly so that your arms were up for only short periods at a time, you wouldn't have a problem. If you continue in this pattern, it is probable that you will do damage to your nerves and/or joints and/or tendons in the long term. What you need to do is train yourself to adjust your position frequently while you sleep. (Most people will, while in deep rem sleep, adjust their position every 10 to 15 minutes on average.)