Is it permissible for a Muslim to sleep on his left side, and is it permissible for him to sleep on his stomach, whether for back comfort or just for sleeping?

It is recommended to sleep on the right side because the heart is located on the left side. If a person sleeps on their right side, they will feel restless and will not sleep deeply, so that their sleep does not become heavy and they miss the night vigil (Qiyam al-Layl). This type of sleep is more beneficial for the heart, whereas sleeping on the left side is more beneficial for the body.

The Prophet, peace and blessings be upon him, forbade sleeping on the stomach, describing it as "a posture Allah dislikes," "a posture Allah does not love," and "the posture of the people of Hellfire." Furthermore, sleeping on the stomach is disliked (makrooh) and unhealthy. In the case of a sick person, it indicates mental confusion and pain in the abdominal areas.

The worst sleeping position is on the back, though lying on it for rest without sleeping is not harmful. Even worse than that is for a person to sleep prostrate on their face.