The phase of circadian rhythms can be shifted by exposure to light. Most patients with winter depression are suggested to have a phase delay in their circadian rhythms. The efficacy of light in treatment of winter depression is thought to be associated with the phase shifting effect of light. In addition to light, melatonin and serotonin can cause phase shifts of the rhythms. The concerted action of these indoleamines may underly the finding that light has an antidepressant effect, independently of time of day or circadian phase of an individual.