The Snowball Effect: Snow cover increases light exposure, suppresses melatonin, and improves alertness in an urban population at northern latitudes

Modern lifestyles increasingly involve indoor activities, which entails considerations for ensuring sufficient light exposure for circadian entrainment and overall well-being. At higher latitudes, where daylight hours are limited in winter, environmental factors such as snow cover may enhance light exposure through its high reflectance. This study investigates the potential effects of snow cover on human biology and behavior in an urban population in Uppsala (59.9° North), Sweden between December and February. Forty-five individuals were monitored for 1 week when there was snow cover and another week without snow cover by using actigraphy, diaries, and collection of morning saliva melatonin samples. Our findings suggest that indoor dwellers at northern latitudes struggle to obtain adequate natural light exposure to sustain mood and alertness during December and January. However, snow cover increases morning light exposure, lowers melatonin levels, and enhances alertness and activity throughout the day. On the other hand, snow cover had minimal influence on sleep-wake cycles, sleep quality, and daytime mood ratings.