The Prophet Mohammed SAWS (pbuh) used to sleep on his right side , as opposed to the right side. There are several reasons why this may have been told to you, depending on your actual health condition. In some situations, sleeping on just the left side can be bad for your heart or it can make sleep better by giving your body a better ability to function.
Sleeping on your left side may be bad for your heart. This is especially true for those who are suffering from chronic heart failure or have had a heart attack. Sleeping on your left side will decrease the sympathetic nervous activity in your body. This activity normally controls blood pressure and the heart rate.
When lying on your right side, you may lower your blood pressure and heart rate, which is good for individuals suffering from heart conditions. Lying on the left side decreases your heart’s ability to function properly.
Doctors recommend that patients with a heart condition, especially chronic heart failure try sleeping on their right side if it is comfortable for the patient.
Avoid the left: Whenever possible, avoid sleeping on the left side of your body. You place a lot of strain on the heart, as well as your lungs, stomach and liver. Some of these organs can press into the heart’s region, making it even more difficult to work properly.
The Prophet SAWS (peace and blessings of Allaah be upon him) used to sleep on his right side, putting his right hand on his right cheek, and this is what the Prophet SAWS (peace and blessings of Allaah be upon him) recommended others to do. According to a saheeh hadeeth narrated from al-Baraa’ ibn ‘Aazib, the Prophet SAWS (peace and blessings of Allaah be upon him) said: “When you go to bed, do wudoo’ as if for prayer, then lie down on your right side…” (Narrated by al-Bukhaari, al-Wudoo’, 239).
So if you sleep on your right side you gain banafit from Allah because it is Sunnah and it is also good for your health.
The Sunnah is to put one’s right hand under one’s cheek. It was narrated that Hudhayfah (may Allaah be pleased with him) said, “When the Prophet SAWS (peace and blessings of Allaah be upon him) went to bed at night, he would put his hand under his cheek, then say, ‘Allaahumma bismika amootu wa ahyaa (O Allaah, in Your name I die and I live).’ Then when he woke up he would say, ‘Al-hamdu Lillaah alladhi ahyaanaa ba’da ma amaatanaa wa ilayhi al-nushoor (Praise be to Allaah who has brought us back to life after causing us to die, and to Him is the resurrection).’” (Narrated by al-Bukhaari, 6314)