You may not be able to control which side you fall asleep on and it may not mean much to you but recent research suggests that it is much better to sleep on one's left side. The reason for this is that it helps improve the work of our digestive system, which is very active while we are asleep.
In ancient times, people had many traditions after their meals including lying down for a while after they ate. (Although according to Jewish tradition one should walk a bit after a heavy meal before repose). Alternative medicine and oriental medicine suggest lying on one's left side after meals, since some major digestive processes occur on the left side of the body starting with the lymph node. The thoracic duct, the largest lymphatic vessel in the human body is full of drained fluids from the lymph nodes. This duct is located on the left side and therefore all proteins in the lymph, all glucose, everything which is bad in your organism that the lymph nodes are purifying goes straight to the left chamber of your heart.
The stomach and pancreas which make digestive enzymes also reside on the left hand side of the body, so when one leans on the left side they hang down naturally, which allows for better digestion, since food moves through the stomach naturally and the enzymes are released as needed rather than all at once which would happen if they had to defy gravity.
Moreover, the small intestine dumps waste through the ileocecal valve (ICV) on the right side of the body into the beginning of the large intestine. The large intestine travels up the right side of your belly the across the tummy, where it dumps waste into the descending colon on the left side.
Sleeping on the left side will allow gravity to encourage the food waste to move more easily from the small intestine into the large intestine through the ICV.
As the night wears on and you continue to sleep on your left side, the waste moves more easily into the descending colon. With the help of gravity and a good night’s sleep on the left the side, the descending colon is full of waste to easily eliminate completely each morning.
Additionally, sleeping on one's left side may aid the aorta which transfers blood into the abdomen. In this mode of sleep the heart is pumping its biggest payload into the descending aorta without needing to send it upwards as well.
Lying and sleeping on the left side is also beneficial for those suffering from esophageal reflux (heartburn).
However some scientists maintain that sleeping on the right side is healthier for heart patients. For those suffering from high blood pressure as well as those who have chronic heart conditions or have suffered a heart attack, it is preferable to sleep on their right side as this reduces heart rate and blood pressure.
In conclusion, we can say that in most cases sleeping on one's left side is more beneficial for one's health, and even after meals it is better to lie on one's left. It is interesting to note that Chazal ordained that one should lean on one's left during the Seder night meal and this would appear to be in consonance with the opinion of alternative medicine mentioned above.