The Impact of Sunlight Variation on Depressive Symptoms Following Treatment for Major Depressive Disorder

Abstract

Introduction: Seasonal variation in sunlight has been implicated in the pathogenesis and maintenance of major depressive disorder (MDD), but has not yet been evaluated as a potential moderator of recovery. Given the high rate of recurrence following treatment for depression, there is a need to identify risk factors that contribute to poor long-term prognosis. The present study seeks to examine the relationship between seasonal sunlight and follow-up outcomes for MDD treatment. Methods: A secondary analysis was conducted on data collected by the NIMH Treatment of Depression Collaborative Research Program. Follow-up data were available from 159 participants who completed the acute treatment phase of the study, which consisted of cognitive behavior therapy (CBT), interpersonal therapy, imipramine, or placebo administered for 16 weeks. Depressive symptom severity, recovery status, and categorical ratings of symptomatology were then measured on four occasions during the 18-month follow-up period. Treatment site-specific sunlight data were collected from the NREL National Solar Radiation Data Base. Multilevel modeling was used to estimate seasonal differences for all outcomes. Results: Sunlight intensity at the time of patient assessment was significantly inversely associated with the severity of depressive symptoms, and the strength of this relationship decreased over the follow-up period. At the baseline (treatment termination) assessment, a one standard deviation increase in sunlight intensity predicted a 1.83-point reduction on the Hamilton Rating Scale for Depression. Symptom severity was significantly greater during winter sunlight conditions (versus summer) at both the baseline assessment and at the 6-month follow-up. At the baseline assessment, the effect of winter-versus-summer sunlight was substantial, equivalent to approximately 4.5 points on the HRSD; the effect was reduced to just over two HRSD points at the 6-month assessment. Exploratory analyses revealed that significant seasonal-typical differences in symptom severity were limited to participants who had been assigned to the CBT and placebo conditions. Sunlight was not significantly associated with recovery status or categorical ratings of symptomatology. Conclusion: Sunlight intensity was inversely associated with depressive symptom severity following psychotherapy, pharmacotherapy, or placebo treatment for depression. This effect persisted for six months post-treatment. Completing treatment during winter-typical sunlight conditions was associated with substantially higher levels of residual depressive symptoms—particularly for participants receiving CBT. Such adverse effects of relative sunlight deprivation could, in turn, increase the risk of negative long-term clinical outcomes, and prolonged or adjuvant treatments for light-deficient patients may be warranted.