Melanopsin, photosensitive ganglion cells, and seasonal affective disorder

► We review recent advances in the understanding of the retinal photopigment melanopsin. ► We review etiological hypotheses for seasonal affective disorder. ► Evidence for a role for melanopsin in seasonal affective disorder is presented. ► Benefits to future research on melanopsin in seasonal affective disorder are described. ► Important factors for future research on melanopsin in seasonal affective disorder are detailed.