Chronic Fatigue Syndrome From the U.S. National Institutes of Health. "While no one knows what causes CFS, for more than a century, doctors have reported seeing illnesses similar to it."
Chronic Fatigue Syndrome: One Man's Recovery By a sufferer, not a medical professional. He lists many possible factors which can produce CFS-like symptoms and reports his own success with light therapy, raising the possibility of a link with Seasonal Affective Disorder (SAD) in his case