In animals, the circadian pacemaker regulates seasonal changes in behavior by transmitting a signal of day length to other sites in the organism. The signal is expressed reciprocally in the duration of nocturnal melatonin secretion, which is longer in winter than in summer. We investigated whether such a signal could mediate the effects of change of season on patients with seasonal affective disorder.
The duration of melatonin secretion in constant dim light was measured in winter and in summer in 55 patients and 55 matched healthy volunteers. Levels of melatonin were measured in plasma samples that were obtained every 30 minutes for 24 hours in each season.
Patients and volunteers responded differently to change of season. In patients, the duration of the nocturnal period of active melatonin secretion 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).
The results show that patients with seasonal affective disorder generate a biological signal of change of season that is absent in healthy volunteers and that is similar to the signal that mammals use to regulate seasonal changes in their behavior. While not proving causality, this finding is consistent with the hypothesis that neural circuits that mediate the effects of seasonal changes in day length on mammalian behavior mediate effects of season and light treatment on seasonal affective disorder.