¹National Taiwan University Hospital, Psychiatry, Taipei, Taiwan, ²National Taiwan University, Institute of Epidemiology and Preventive Medicine, College of Public Health, Taipei, Taiwan, ³National Taiwan University College of Medicine, Department of Psychiatry, School of Medicine, Taipei, Taiwan, ⁴Wang-Fang Hospital, Taipei Medical University, Department of Psychiatry, Taipei, Taiwan, ⁵Taipei Medical University, Department of Psychiatry, School of Medicine, Taipei, Taiwan, ⁶Taipei City Hospital, Songde branch, Taipei City Psychiatric Center, Taipei, Taiwan, ⁷Fu Jen Catholic University, School of Medicine, College of MedicineTAatt, New Taipei City, Taiwan, ⁸National Taiwan University, Department of Public Health & Institute of Epidemiology and Preventive Medicine, College of Public Health, Taipei, Taiwan

Background: Major depressive disorder (MDD) and bipolar disorder (BD) often exhibit mood fluctuations linked to seasonal changes. Vitamin D level and light exposure has been proposed to influence the mood symptoms, while the interaction between them has not been studied.

Aims & Objectives: This study investigates seasonal variations in light exposure and vitamin D levels in individuals with mood disorders, and explores the interaction between mood symptoms, vitamin D level and light exposure.

Method: This longitudinal study included individuals aged 20–70 diagnosed with MDD or BD according to DSM-IV criteria, along with healthy controls recruited from four outpatient clinics. Mood symptoms were assessed in the mood disorder groups using the Hamilton Depression Scale (HAMD) and Young Mania Rating Scale (YMRS). Vitamin D level was examined in each season. Light exposure was obtained by actigraphy for a continuous week in each season. Cosinor analysis was examined for the seasonality of vitamin D level and mood symptoms. We fitted generalized estimating equations (GEE) using an exchangeable working correlation structure to account for within-subject correlations across four seasonal measurements. To appropriately model the cyclical nature of seasonal variation, season was represented using its sine and cosine transformations.

Results: A total of 161 participants were included in the study. No significant group differences were observed in either vitamin D level or light exposure across seasons. Significant seasonal variation in light exposure was observed in most diagnostic groups, except for MDD and BD with SAD. Vitamin D levels showed significant seasonal variation in HC (β = 2.074, p = 0.03) and BD (β = 1.958, p = 0.004), but not in MDD (β = 1.656, p = 0.119). No significant seasonal fluctuation was found in mood symptom severity across diagnoses, including participants with SAD. Correlation analyses revealed that both vitamin D levels (r = -0.125, p = 0.017) and light exposure (r = -0.142, p = 0.028) were negatively correlated with HAMD scores, suggesting that higher light exposure and higher vitamin D levels were associated with fewer depressive symptoms. As for the GEE, a significant interaction (light × vitamin) in HAMD was observed only in MDD ((β = -0.001, p = 0.048) but not in BD (β = 0.001, p = 0.176), indicating that the moderating effect of vitamin D on the light exposure and depressive symptoms.

Discussion & Conclusions: These results suggest that both vitamin D level and light exposure was negatively associated with the severity of depression. Moreover, the seasonal variation of vitamin D level was different in MDD from BD. Besides, the beneficial effect of light exposure on depressive symptoms was stronger at higher vitamin D levels only in MDD but not in BD. The physiological effect of vitamin D level in MDD should be further explored.