The most common reason is osteoarthritis which causes a patient to experience aching, stiffness, and eventual loss of mobility within the knee joint. Inflammation may or may not be present. The pain may be severe at times, followed by periods of relative relief. It often worsens after extensive use of the knee and is more likely to occur at night than in the morning. Stiffness tends to follow periods of inactivity, such as sleep or sitting and can be eased by stretching and exercise. Pain also seems to increase in humid weather. As the disease progresses, the pain may occur even when the joint is at rest and can keep the sufferer awake at night.