Functional somatic syndromes are physical illnesses without an organic disease explanation and devoid of demonstrable structural lesion or established biochemical change. The degree to which these syndromes have been characterized as unique entities is variable. In 1989 two Harvard Medical School researchers J.I.Hudson and H.G.Pope Jr. published an analysis of the association between fibromyalgia and psychopathology. They formulated three explanatory hypotheses: fibromyalgia is the cause of psychopathology; fibromyalgia is the effect of psychopathology; fibromyalgia and psychopathology are the result of a common underlying morbid process. For fibromyalgia, Robert Kellner suggested that physical disease, psychopathology and low serotonin concentration cause abnormalities in non-rapid eye movement sleep. Kellner also addressed the tendency for clustering of functional somatic syndromes, a prominent feature of the work of Hudson & Pope and D.Stewart. Kellner's work has been continued and expanded in the latest contribution to the evolution of ideas regarding the etiology of these syndromes.