In ancient times, some monks would lie down after they ate for a few minutes to improve digestion, which sleep can actually help with if done correctly.
Ancient Eastern medicines also suggest sleeping on the left side because the left side of the body is the dominant lymphatic side. The bulk of the body's lymph fluids drain into the thoracic duct which is on the left side and during this process glucose and other metabolites and waste products are purified and then drained into the left side of the heart.
People who experience heartburn may also find some relief from sleeping on their left side because when we sleep on the left side, our stomachs are seated below the entrance from the cardiac sphincter which can help prevent discomfort from heartburn.
However, it is wise to listen to our body, if we feel comfortable and at peace sleeping in a particular position then we should stick with it. The body is always communicating with us, if we just listen to what it has to say.