Hi @valb68 I have PMR and GCA. Morning pain and stiffness is one of the key symptoms of PMR "The musculoskeletal hallmark of PMR is proximal, severe, and symmetrical morning and even day long stiffness, soreness and pain in the shoulder, neck and pelvic girdle. Fifty percent of patients with GCA share this characteristic proximal pain syndrome." (Reference - Cecil and Goldman's Textbook of Medicine).
If you do not have pain, are you taking prednisone? On the right dosage, the stiffness and other symptoms would lessen or disappear. By the way PMR and GCA are autoimmune diseases and melatonin comes with a warning not to take it if you have an autoimmune disorder.