Prophet Muhammed (peace be upon him) advised us 1400 years ago to start our sleeping mode on the right side,
Scientists say that sleeping on the
left side is not favored, as the heart would be under the pressure of the right lung, the biggest of the two lungs, and this would affect its contraction especially if the person is old. Also they assure that the full stomach which itself is under the pressure of the heaviest organ in the digestive tract, the liver, which delays its emptying, would also impose a pressure on the heart.
Hence they advise people to sleep on the right side especially for patients by any heart disease.