Natural bright light exposure in the summer and winter in subjects with and without complaints of seasonal mood variations

Background: Considering the success of bright light therapy in seasonal affective disorders, it was suggested that seasonal mood disorders are triggered by decreased exposure to bright light in the winter; however, no previous studies have used objective measures to assess seasonal patterns of bright light illumination in subjects with seasonal mood variations. Methods: Eleven subjects reporting seasonal mood variations and 8 control subjects had their levels of natural bright light (BL) exposure measured for 5–6 days with an ambulatory monitor during both the summer and winter, at a latitude of 45°31′N. Results: Both groups received significantly more BL in the summer than in the winter, but there was no difference between the two groups for the pattern of BL exposure, including total duration, daily distribution, and amplitude of seasonal variation. Conclusions: These results suggest that complaints of seasonal mood variations are not caused by a differential pattern in bright light exposure compared to normals. It is possible, however, that some individuals are more sensitive than others to variations in natural bright light. Whether an increased vulnerability is due to a more fragile affective state or to a lower sensitivity to light remains to be determined.