 Attempts to construct an action spectrum for suppression of melatonin secretion and for resetting the circadian pacemaker suggest that 446-477 nm wavelengths are most potent Brainard et al., 2001; Lockley et al., 2003, which is inconsistent with rod or cone mediation of the responses Brainard, 2004. Discovery that a melanopsin-containing system of retinal ganglion cells serves as a primary afferent to the hypothalamic circadian pacemaker Provencio et al., 2000; Gooley et al, 2001; Hattar et al, 2002 suggests a possible basis for the potency of violetblue light on these neuroendocrine circadian endpoints and raises the possibility that light of this wavelength also might be uniquely potent for treatment of SAD Brainard, 2004. 