It could be your sleep position. Try sleeping on back, with arms to your side, your head can be turned to one side or the other. Don't cross your legs, or sleep on your side, or put your arms over your head.
How old are you? When you have problems with your heart you are tired because your heart has to work harder. It's not just old people who have heart diease. Do you have it in your family? My daughter had a heart attack and had 5 stents two weeks before she was 40.