Sleeping on his right side is the best manner of sleeping, and is in accordance with the Sunnah of the Prophet (peace and blessings of Allaah be upon him), and is beneficial for the body.
It was narrated by Ahmad and Abu Dawood (5040) that Takhfah ibn Qays al-Ghifaari said:
When I was lying on my stomach in the mosque just before dawn, a man nudged me with his foot and said: “This manner of lying is hated by Allaah.” I looked and saw that it was the Messenger of Allaah (peace and blessings of Allaah be upon him).
Ibn Majaah (3724) narrated from Ibn Takhfah al-Ghifaari that Abu Dharr said:
The Prophet (peace and blessings of Allaah be upon him) passed by me and I was lying on my stomach. He kicked me with his foot and said: “O Junaydib! This is how the people of Hell lie.”
The ahaadeeth which speak of the prohibition on sleeping on one’s stomach vary in their degree of soundness. Some scholars classed them as da’eef (weak) such as al-Bukhaari, al-Daaraqutni, Ibn Abi Haatim and Ibn Rajab, and others classed them as saheeh or hasan, such as Ahmad Shaakir in Tahqeeq al-Musnad (14/248-249) and al-Albaani in Saheeh Abi Dawood.
The scholars of the Standing Committee said that it is makrooh to sleep on the stomach, because of the report narrated by Abu Dawood from Takhfah ibn Qays al-Ghifaari.
From a medical point of view, sleeping on the right side is the healthy position, because the left lung is smaller than the right, so the heart is under less pressure and the liver is settled and not suspended, and the stomach is resting comfortably on top of it, which as we have seen makes it easier to eliminate the food that is in it after it has been digested. Sleeping on the right side is also regarded as the best medical procedure that makes it easy for the tubes of the left lung to expel mucus discharges.
As for sleeping on the left side, it is not acceptable, because in that case the heart will be beneath the pressure of the right lung, which is larger than the left, and this will affect its function and make it less active, specially in those who are elderly. The full stomach will also be under pressure, which will increase the pressure on the heart and liver – which is the heaviest organ – which will not be fixed, rather it will be hanging, as it is located on the right side, so it will put pressure on the heart and on the digestive tract which will delay elimination.
Experiments have proven that the passage of food from the stomach to the intestines takes between 2.5 and 4.5 hours, if a person is sleeping on his right side, but it takes 5 to 8 hours if he sleeps on his left side.
See: al-Tibb al-Nabawi fi Daw’ al-‘Ilm al-Hadeeth by Dr. Ghiyaath al-Ahmad, and al-Nawm ‘ala al-Jihah al-Yumna by Dr. Ibraaheem al-Raawi.