Morning stiffness is caused by fluid or edema around the affected joint during sleep or reduced activity, causing tension in the muscle tissue around the joint, resulting in swelling and pain or stiffness, and with muscle contraction, the edema is absorbed by the lymphatic vessels and small veins, and the morning stiffness is relieved. Although both rheumatoid arthritis and osteoarthritis can lead to morning stiffness, there are differences, as the duration of morning stiffness in rheumatoid arthritis is significantly longer than that in osteoarthritis.