Sleeping on either side or in any position is okay for your health.
But if you want to improve digestion then it would be best to sleep on your left side.
Sleeping on your left side not only helps with digestion but also helps improve blood circulation which is a reason why doctors advise that when a woman is pregnant she should sleep on her left side during the pregnancy.
Sleeping on your left side also takes some weight off the heart which helps keep your heart healthy while improving blood circulation.