 Discovery in the late 1970s that bright light affects neuroendocrine rhythms such as melatonin secretion, and can reset the circadian pacemaker in humans, paved the way for the discovery that seasonally recurrent fallwinter depressions--christened Seasonal Affective Disorder SAD--can be treated with bright light exposure Lewy et al., 1980; Rosenthal et al., 1985. It was reasoned that seasonal rhythms of mood in humans, like other seasonal physiological cycles in mammals, could be regulated by the biological clock. A series of studies has demonstrated that bright light exposure does produce therapeutic effects in SAD patients beyond the level achieved by placebos Eastman et al., 1998; Lewy et al., 1998; Terman et al., 1998 and that retinal exposure is required for efficacious treatment Wehr et al., 1987; Koorengevel, 2001. 