Winter has shorter days and less sunlight.
Scandinavian countries have done a lot of research on light effect behavior. They're a softer quality and much better for your eyes and spirit. Along with there types of light bulbs, the use of aromatherapy, ginseng, and massage have been proven to increase people's spirits and energy during longer periods of inactivity. Using all natural, non-synthetic products allows the body to feel rejuvenated and alive. Smell is a much more subliminal thing.