A mechanism of action underlying the antidepressant effect of light.

In patients with winter seasonal affective disorder (SAD), delayed and reduced responses to corticotropin-releasing factor (CRF) have been observed. Bright light treatment has been shown to normalize these responses. In depressed patients, there is increased CRF activity in the evening during the normally quiescent period between 19.30 and 22.00 hours. In patients with winter SAD, the level of subjective sleepiness is increased in the evening between 20.00 and 21.00 hours. In the latter group of patients, the CRF activity may be increased in the evening and associated with the increased level of subjective sleepiness. This increased activation is suggested to be normalized by bright light treatment, acting primarily on neurons of the paraventricular nucleus of the hypothalamus.