Leo Sher, M.D.

Our research report, “A circadian signal of change of season in individuals with seasonal affective disorder” was published in the December 2001 issue of the Archives of General Psychiatry (1). 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 (SAD).

The duration of melatonin secretion in constant dim light was measured in winter and in summer in 55 patients with SAD and 55 matched healthy volunteers. Healthy volunteers matched with patients by age and sex showed no evidence in their Structured Clinical Interviews of current or past Axis I psychiatric disorders and had no first-degree relatives with histories of psychiatric illnesses. 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 SAD 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 a 38-minute change in duration of the circadian pacemaker’s day length signal may seem small, a change in the photoperiod of this magnitude is sufficient to elicit behavioral changes in other mammals. Our 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 SAD.

Reference

1. Wehr TA, Duncan WC Jr, Sher L, Aeschbach D, Schwartz PJ, Turner EH, Postolache TT, Rosenthal NE. A circadian signal of change of season in patients with seasonal affective disorder. Arch Gen Psychiatry. 2001 Dec;58(12):1108-14. doi: 10.1001/archpsyc.58.12.1108.