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

LIGHT IS NATURE'S STIMULANTThe upbeat feeling we get on a bright sunny day is no accident. This feeling is the result of light triggering our bodies to release "active" hormones to help improve our mood and energy levels.

Winter can be a challenge for many because the shorter days and longer nights leave us simply "light-deprived". Without the normal light stimulation, we often feel down, lethargic and unhappy.

Light therapy provides a natural boost that helps you feel better, increases energy and even helps you sleep. Using the right wavelength of light, you can trigger your active hormones naturally, boosting your mood and overcoming those down feelings, whatever the season.