Changes in the duration of nocturnal melatonin secretion that are induced by seasonal changes in the length of the night trigger many of the dramatic seasonal changes in behavior that occur in animals. It has been hypothesized that melatonin plays a similar role as chemical mediator of the effects of season on mood in patients with seasonal affective disorder (SAD). For such a hypothesis to be tenable, it would be necessary to show that duration of nocturnal melatonin secretion changes on a seasonal basis in patients with SAD. We have preliminary evidence that the duration (and amplitude) of nocturnal melatonin secretion increases in winter and decreases in summer in healthy women, but not in healthy men. This gender difference, if confirmed, might explain why SAD is more prevalent in women than men. Interestingly, all of five men with SAD that we have studied to date exhibited lengthening of melatonin secretion in winter, as occurs in healthy women. However, the response of women with SAD is less consistent. The purpose of the present protocol is to extend and confirm our finding of gender differences in the response of melatonin secretion to change of season in healthy individuals, and to determine whether and to what extent this response occurs in patients with SAD.