Especially for those of us who already have to deal with shoulder pain, the answer is a resounding “yes.”
As a quick experiment, lie on your side. Think about the positioning of your shoulder as you lie on top of it. Most people will unconsciously position the body so that the bottom shoulder is forced into a forward position. This allows the body to avoid planting weight directly down through the shoulder, which can cause immediate numbness and pain in the arm.
In the long term, the relative forward position of the shoulder joint with side-sleepers feeds into a basic mechanical fault. When the head of the humerus, or shoulder bone, is chronically pushed forward, the joint capsule that surrounds it tends to accommodate to this forward position. The front of the capsule tends to get over-stretched from the bone constantly being pushed forward, and the back of the capsule becomes tighter because the head of the humerus is not resting in a position that places any normal force on it.
The resulting restriction in posterior glide can cause a painful “impingement” of the shoulder rotator cuff tendons against structures that are normally not in the way.
The general consensus of the medical community is that sleeping on your back (face upwards) places a lesser amount of stress on both the shoulder and the neck. I would particularly advise against side sleeping for people who have had shoulder surgery, thoracic outlet syndrome, or rotator cuff tendonitis.
Read More About Shoulder Issues
• What Does a Shoulder Labral Tear Feel Like?
• How Do I Stabilize an Unstable Shoulder?
• When Can I Start Working Out After Shoulder Surgery?
• How Do I Deal With A Shoulder Dislocation?
• Why Won’t My Shoulder Rotate?
• Managing Expectations When Recovering from a Labral Repair