Recently a Dr. Gupta video was posted on blood pressure. The two videos below give a bit more background. Note they were recorded at different times so the information is a bit different in each. In the first one, Dr. Bergman mentions why laying on our left side affects artery blood flow. Many people on this site have mentioned when they lay on their left side, the heart is affected. Maybe this blood flow change impacts the heart.
Blood Pressure and Cholesterol