 Recurrent fallwinter major depression known as Seasonal Affective Disorder SAD is a prevalent and disruptive disorder whose pathophysiological basis is unknown, but several hypotheses attribute a causal role to the circadian timing system. Bright white light exposure via the retina has been shown to reverse the symptoms of SAD. Recent physiological studies demonstrated the existence of retinal ganglion cells capable of transducing light input to the retinohypothalamic tract, the primary circadian afferent in humans. This retinohypothalamic system appears to be maximally sensitive to light in the 446-477nm violetblue range. 