A circadian signal of change of season in patients with seasonal affectivedisorder

Background: In animals, the circadian pacemaker regulates seasonal changes in behavior by transmitting a signal of day length to other sites in the or ganism. The signal is expressed reciprocally in the duration of nocturnal m elatonin secretion, which is longer in winter than in summer. We investigat ed whether such a signal could mediate the effects of change of season on p atients with seasonal affective disorder. Methods: The duration of melatonin secretion in constant dim light was meas ured in winter and in summer in 55 patients and 55 matched healthy voluntee rs. Levels of melatonin were measured in plasma samples that were obtained every 30 minutes for 24 hours in each season. Results: Patients and volunteers responded differently to change of season. In patients, the duration of the nocturnal period of active melatonin secr etion was longer in winter than in Summer (9.0 +/-1.3 vs 8.4 +/-1.3 hours; P=.001) but in healthy volunteers there was no change (9.0 +/-1.6 vs 8.9 +/ -1.2 hours; P=.5). Conclusions: The results show that patients with seasonal affective disorde r generate a biological signal of change of season that is absent in health y volunteers and that is similar to the signal that mammals use to regulate seasonal changes in their behavior. While not proving causality, this find ing is consistent with the hypothesis that neural circuits that mediate the effects of seasonal changes in day length on mammalian behavior mediate ef fects of season and light treatment on seasonal affective disorder.