I too experience Chronic Fatigue. My doctors think it is due the all the stress I have been under for years, wearing my body down. It is mostly due to the PTSD though so I am not sure about its link with other disorders. With the PTSD my body has been in 'fight or flight' mode, producing adrenaline for too long without respite. There is a theory that this fatigues the body and leaves too many chemicals in our system than wears a body down. Stress is the main cause so reducing stress as much as possible is the key to reducing this type of fatigue. Since I have been able to reduce my stress, and paced my activities to what I can manage without exhausting myself each day I have begun to feel better and have more energy. I hope that makes sense.