Are Deficient Retinal Photoreceptor Renewal Mechanisms Involved in the Pathogenesis of Winter Depression?

To the Editor.—  Seasonal affective disorder (SAD)¹ is a recurrent form of depression associatedwith the length of daylight or photoperiod. It can be effectively treated by exposure to bright light.² This novel therapy developed from circadian models of seasonal behavior in mammals and, by analogy, has thus been hypothesized to be a circadian intervention. One study suggests that, indeed, the response to light is mediated by the eye,³ but none of the proposed mechanisms has received unequivocal empirical support, whether it is that the therapeutic effect is mediated by a phase shift induced by a light pulse, by an augmentation of circadian amplitude, or an increase in number of photons absorbed.We suggest that SAD could be tied to a deficiency in retinal photoreceptor renewal mechanisms. Since retinal response to light is the first step in circadian rhythm regulation, understanding it is therefore crucial in considerations of