hi, my theory is that
as the day wears on and the scapular stability worsens and the scapular stabilizers attached to the thoracic spine weaken, the pectoralis minor becomes more dominant and the shoulder rolls forward more and the brachial plexus is compromised under the pec minor (pec minor syndrome). When you sleep, as long as you dont sleep on the affected side the pain could subside because the thoracic stabilizers can recover and the posture will improve . if i tried to wipe our dining table of an evening i would get pain just like electricity, flying up from my hand as i leaned forward and stretched out. if i taped my shoulder back for the day there was a significant improvement

regards Towelhorse