Scientists found out that cerebrospinal fluid gets rid of toxins better if we sleep on our side. It reduces headaches and the risk of getting Alzheimer's and Parkinson's diseases. Additionally, sleep on the left side eases heartburn but increases the pressure on the liver.

In case of digestive issues doctors recommend sleeping on the left side. It helps improve the bloodstream, relieving the gullet and the stomach.
To keep your back straight when you sleep on your side, put a hard and small pillow between your legs: it'll reduce the pressure on joints. Put a soft and big pillow under your head.
In case of high blood pressure and problems with blood circulation it's recommended to sleep on your right side: it'll reduce the pressure on your heart.

How to Sleep in Your Favorite Position Without Harming Your Health