Painful muscle areas. Difficulty remaining asleep. Possibly an imbalance in brain chemicals or an autoimmune disorder. Until recently, this was thought to be a psychological disorder, but this is no longer the wide-spread belief.
Research continues into the cause.
Exposure to dampness or cold. Medical history of disorders that produce joint inflammation, such as rheumatoid arthritis or polyarteritis. Fatigue or overwork. Get adequate sleep. General conditioning exercises.