Phototherapy and its mechanisms of action in seasonal affective disorder.

Eleven depressed patients with seasonal affective disorder completed three different treatments of 1 week each given in a balanced order with a 1-week withdrawal between each week of treatment. The three treatments were photoperiod extension with bright light, or with dim light, and light augmentation with bright light without a change in photoperiod. Most patients improved on all treatments, with a trend in favour of bright light over dim. Only light augmentation was significantly better than dim light and was also superior to photoperiod extension. These findings do not replicate earlier studies and, as the most successful treatment involved no change in photoperiod, they suggest that modification of melatonin secretion may not be the mechanism of action of phototherapy.