I cut and pasted this excerpt from an interesting website talking about Light Therapy for Seasonal Affective Disorder (SAD):

Personal Light Therapy

It is believed that over 75 million people in the U.S. are light deprived -- and are paying for it with their health. Referred to as Seasonal Affective Disorder (S.A.D.), the most common symptoms are low energy, low mood, and poor quality of sleep. Additionally, irritability, difficulty concentrating, carbohydrate cravings, and reduced libido can be the effects of not getting enough bright light on a daily basis. If what I'm describing sounds like you, or someone you love, think about it: we spend most of our days indoors, where there is little or none of the sun's life-giving bright light -- and none of its beneficial wavelengths. Jet lag, shift work, the short days of winter, or anything that disrupts the body's clock, often result in bright light deprivation symptoms, too.