A person can dislocate their shoulder while sleeping, though it is uncommon. A shoulder dislocation occurs when the head of the humerus (upper arm bone) completely separates from the glenoid socket (part of the shoulder blade). This joint is the most frequently dislocated major joint due to its extensive range of motion. For a dislocation to happen without significant trauma, a pre-existing structural weakness in the joint is necessary.
The Critical Role of Underlying Shoulder Instability
Sleep does not possess the force required to dislocate a healthy shoulder; it merely exposes an already vulnerable joint to mechanical stress. The most common precursor to a nocturnal dislocation is a history of previous traumatic injury. Once the shoulder has dislocated, the surrounding soft tissues that provide static stability are compromised, making the joint prone to repeated episodes.
The fibrous capsule surrounding the joint and the labrum—a rim of cartilage around the socket—can be stretched or torn during the initial injury. A labrum tear, often called a Bankart lesion, removes a physical barrier that helps keep the humerus centralized. This structural damage means that a simple, non-forceful movement can be enough to push the humeral head out of its proper alignment.
Generalized joint hypermobility, or natural ligamentous laxity, is another underlying factor. Individuals born with naturally loose ligaments are prone to multi-directional instability in the shoulder. This inherent looseness means the joint relies heavily on muscle strength for dynamic stability, which is reduced during sleep.
How Sleep Positions Contribute to Dislocation
The mechanics of a sleep-related dislocation rely on muscle relaxation combined with specific limb positioning. During deeper stages of sleep, particularly Rapid Eye Movement (REM) sleep, the body experiences muscle atonia, or severe muscle relaxation. This relaxation removes the active muscular stabilization provided by the rotator cuff, which normally keeps the humerus centered in the shallow glenoid socket.
When muscular support is absent, the vulnerability of the unstable joint is exploited by compromising positions. The most common hazardous position involves the arm being overhead, known as abduction and external rotation. This position places maximum strain on the anterior stabilizing structures, which are often damaged in a previous dislocation. The leverage created by the arm’s weight can then force the humeral head forward and out of the socket.
Lying directly on the side can also contribute to instability by applying sustained compression to the joint. If the arm is in an awkward or rotated position, this prolonged pressure can gradually push the loose joint out of place. The combination of full body weight pressure and lack of conscious muscular resistance creates the necessary conditions for a subluxation or full dislocation.
Recognizing the Signs and Seeking Immediate Care
Waking up with a dislocated shoulder is a painful experience that requires immediate attention. The distinct symptoms are sudden, severe pain and a visible deformity of the shoulder joint. The normal round contour of the shoulder may appear flattened or squared off, and a bump might be noticeable near the joint.
A person with a dislocated shoulder will be unable to move the arm, and any attempt will cause a sharp increase in pain. It is common to experience numbness, tingling, or weakness in the arm or hand, which indicates possible nerve or blood vessel compression. The axillary nerve, which runs near the joint, is the most frequently injured structure.
A person should never attempt to reduce, or “pop,” the shoulder back into place themselves. Attempting self-reduction can cause further damage to the surrounding soft tissues, nerves, or blood vessels. The immediate action is to immobilize the arm in the position it is found, perhaps using a makeshift sling, and seek emergency medical care. Only a medical professional, using proper imaging and sedation, should perform the reduction.