Seasonal variation of depressive symptoms in unipolar major depressive disorder

Abstract

Objectives: Retrospective and cross-sectional studies of seasonal variation of depressive symptoms in unipolar major depression have yielded conflicting results. We examined seasonal variation of mood symptoms in a long-term prospective cohort - the Collaborative Depression Study (CDS).

Methods: The sample included 298 CDS participants from five academic centers with a prospectively derived diagnosis of unipolar major depression who were followed for at least ten years of annual or semi-annual assessments. Generalized linear mixed models were utilized to investigate the presence of seasonal patterns. In a subset of 271 participants followed for at least 20 years, the stability of a winter depressive pattern was assessed across the first two decades of follow-up.

Results: A small increase in proportion of time depressed was found in the months surrounding the winter solstice, although the greatest symptom burden was seen in December through April with a peak in March. The relative burden of winter depressive symptoms in the first decade demonstrated no relationship to that of the second decade. The onset of new episodes was highest October through January, peaking in January.

Conclusions: There exists a small but statistically significant peak in depressive symptoms from the month of the winter solstice to the month of the spring equinox. However, the predominance of winter depressive symptoms did not appear stable over the long-term course of illness.

Figure 1

This figure depicts the mean proportion of days per calendar month each subject spent with clinically significant depressive symptoms as operationally defined by a score of ≥3/6 on the major depression scale or 3/3 on the minor depression LIFE scale.

Figure 2

Scatter plots of seasonal indices for the first and second decades of follow-up for each participant with at least 20 years of follow-up (N=271). The seasonal index represents the ratio of the proportion of weeks with clinically significant symptoms during the observed peak (December-April) relative to the remainder of the year. A seasonal index of 1 would suggest an equal proportion of time spent with clinically significant depressive symptoms during the winter months relative to the rest of the year. The top panel includes the entire sample (rho= −0.03, p=0.59) and the lower panel includes the mostly untreated sample (N=147, rho= −0.01, p=0.90).

Figure 3

Timing of relapse into minor or major depressive episodes in persons with unipolar major depression. There were a total of 602 new episodes of minor depression and 579 new episodes of major depression in the sample. Depressive episodes were most likely to begin in January (11.2%) and least likely to begin in July (6.4%).