Lying on the left side and sleeping on the stomach will oppress the heart and affect cardiopulmonary function. Sleeping on your back will oppress blood vessels and cause poor blood circulation.

The heart is located on the left side of the human chest. Sleeping on the right side can make more blood flow to the right side of the human body, thus reducing the burden on the heart and benefiting the rest of the heart. Secondly, the liver is the "chemical factory" of the human body, located in the upper right abdomen. Lying on the right side will make more blood flow through the liver, which is beneficial to the health and metabolism of the liver itself. There are also people whose stomach opens to the duodenum and whose small intestine opens to the large intestine, all of which open to the right. Lying on the right side is beneficial to the normal operation of food in the stomach.

So most people like to sleep on their side.