Fatigue is a frequent complaint found in conjunction with jaw dysfunction. There is considerable medical literature which supports a connection between chronic fatigue syndrome and dysfunction of the trigeminal nerve(nerve to the jaw muscles)- in particular see articles by Jay Goldstein, MD (Chronic fatigue: the limbic hypothesis). Basic brain research has demonstrated that there are fatigue centers in the reticular formation of the brain stem that can be activated by the trigeminal nerve. Also, the trigeminal nerve is a major input into the limbic brain, which is known to be dysfunctional in Chronic Fatigue Syndrome. Since the trigeminal nerve is a predominant effector of Substance P levels in the body, this would also account for fibromyalgia and why it is often seen in conjunction with CFS.
The trigeminal nerve is known to innervate the blood supply to the cranium, which has also been shown to be abnormal in CFS (Semin Arthritis Rheum. 2000 Oct;30(2):79-86. The capnography head-up tilt test for evaluation of chronic fatigue syndrome. Naschitz JE, Rosner I, Rozenbaum M, Gaitini L, Bistritzki I, Zuckerman E, Sabo E, Yeshurun D. Source Department of Internal Medicine A, Bnai Zion Medical Center and Bruce Rappaport Faculty of Medicine, Technion-Israel Institute of Technology, Haifa, Israel.). This same complex (trigeminovascular complex) is known to be involved with headaches, a common finding in CFS.
Clinical research that I have been involved in for over 25 years indicates that jaw orthopedic therapy is about 50 % effective at eliminating all symptoms and about 80 % effective at relieving substantial symptoms of chronic fatigue and fibromyalgia. The primary mechanism by which jaw orthopedic therapy works I believe is by effecting neuropeptide levels.