After elucidating the components and theory of neuroticism (N) as well as of psychosomatic complaints and their relationships to personality dimensions and to psychosomatic diseases, comparisons are performed between patients suffering from fibromyalgia syndrome (FMS) or related pain diseases with healthy subjects scoring high on personality dimensions related to neuroticism. FMS and pain patients score high on depression, anxiety, and experience of stress although questionnaire scores on depression are higher in subjects not exhibiting somatic features of the disease. High subjective pain sensitivity and low thresholds for pain perception are also common features in high N subjects and FMS patients. On the endocrinological level cortisol responses to challenge tests with CRH as well as prolactin responses to TRH are higher in FMS patients than in high N healthy subjects indicating an endocrinological difference. A common feature, however, is the lack of adaptability in the two groups, since neurotics are in particular characterized by a low capacity to shift their behavior from one state to the other (waking-sleeping, working-relaxing), to re-adapt to baseline levels after endocrinological or physiological stress responses, or to adjust to conditions of shift work. This is reflected by chronobiological disturbances in FMS patients and could also explain their maintainance of pain perception, because they are incapable of correcting conditioned pain-producing muscle tension.