More prominent reactivity in mood than activity and sleep induced by differential light exposure due to seasonal and local differences

The aim of this study was to investigate how mood, work time, light exposure, activity, and sleep indices are affected by the differences of latitude and season in healthy volunteers. Twenty-four subjects (38.92±11.32 yrs) in Rochester, Minnesota, USA (latitude 44°1'N) and 30 subjects (47.03±16.32 yrs) in San Diego, California, USA (latitude 32°43'N) completed a 1 yr protocol measuring daily logs including daily work time and sleep diary; the Center for Epidemiologic Studies Depression Scale (CES-D); the eight-item atypical subscore of the Structured Interview Guide for the Hamilton Depression Rating Scale, Seasonal Affective Disorder version (SIGH-SAD); and actigraphic measures of light exposure and sleep for one-week in each solstice of summer and winter. Higher scores of CES-D (p=0.038) and the eight-item atypical subscore of SIGH-SAD (p=0.009), and longer indoor (p=0.001) and shorter outdoor (p=0.002) work times were observed during winter than summer only in Rochester, with no differences in San Diego. The mesor of light was decreased in both Rochester (p≤0.001) and San Diego (p=0.004) during winter compared to summer, with no differences in the mesors (24 h means) of activity and sleep. The eight-item atypical subscore of SIGH-SAD was significantly negatively correlated with the mesor of light (p=0.034). Sleep indices showed no significant differences between two locales or two seasons. A more prominent depressive mood in Rochester than San Diego during winter can be explained by decreased light exposure of healthy subjects in Rochester. Despite a significant difference of mood and light exposure between the two seasons in Rochester, there were no differences in activity or sleep. Therefore, mood might be more reactive than activity and sleep in the seasonal variation induced by differential light exposure.