The shortening of the photoperiod may alter gene expression in winter depression

Winter depression is characterised by recurrent major depressive episodes with a seasonal (winter) pattern. Reduced exposure to daylight due to shortening of the day has been hypothesized to trigger off the onset of a depressive episode. Daily exposures to light during the dark period have been used as the treatment of choice for winter depression. A genetic predisposition for high responsiveness to light is thought to exist in patients with winter depression. Altered gene expression may contribute to the unique sensitivity and mediate the antidepressant effects of light.