Effects of morning phototherapy on circadian markers in seasonal affective disorder

Background The suppression of melatonin by light at 00.30 hours has been shown to be greater in winter than in summer in patients with seasonal affective disorder (SAD) but not in matched normal controls. Method In this study 12 patients with SAD and 12 matched normal controls were exposed to morning light therapy in the winter. Melatonin profiles and sensitivity to light were measured before and after treatment. Results The SAD but not the normal group showed a phase advance of melatonin rhythms in response to phototherapy. There was an association between phase position and phase shift in the SAD but not in the normal group. Conclusions There may be instability of circadian rhythms in SAD mediated by a high-amplitude phase response curve, rather thana fixed phase abnormality as had been previously suggested. This instability may be secondary to impaired serotoninergic function in the afferent pathways to the suprachiasmatic nuclei.