Sixteen depressed patients with seasonal affective disorder participated in a double-blind crossover study comparing the antidepressant effects of 2 hours of early morning and 2 hours of early afternoon therapy with bright light. They responded equally well to both treatments. These results suggest that the antidepressant effects of phototherapy in seasonal affective disorder do not depend on its capacity to extend day length (photoperiod) and are not likely to be due to a shift in the timing of circadian rhythms. These findings have practical implications for the administration of phototherapy in the treatment of seasonal affective disorder.