Generally, sleeping on your left side will reduce digestion issues such as acid reflux. This is because in this sleeping position, your stomach is below your esophagus. If you sleep on the right, the muscles between your stomach and esophagus relax leading to reflux. On the left, gravity can return any escaped digestion acid back to your stomach, while this process is significantly slower if you sleep on your right.  Leaked stomach acid can return to your stomach quicker than when on your right side, which is why the left is usually the best side to sleep on to avoid acid reflux. According to Sleepscore Labs, left side sleeping is preferred to the right side, although your back is the preferred sleeping position for reducing heartburn because your head is elevated above your chest. It also claims that back sleeping is the most healthy sleeping position. Stomach sleeping is the worst since it compresses your stomach, forcing the acid into your esophagus.