Antidepressant and Circadian Phase-Shifting Effects of Light

TLDR

Bright light suppresses nighttime melatonin production, whereas ordinary indoor light does not. Morning bright light improves winter depression and advances melatonin onset, while evening light has no effect, indicating timing is critical.

Abstract

Bright light can suppress nighttime melatonin production in humans, but ordinary indoor light does not have this effect. This finding suggested that bright light may have other chronobiologic effects in humans as well. Eight patients who regularly became depressed in the winter (as day length shortens) significantly improved after 1 week of exposure to bright light in the morning (but not after 1 week of bright light in the evening). The antidepressant response to morning light was accompanied by an advance (shift to an earlier time) in the onset of nighttime melatonin production. These results suggest that timing may be critical for the antidepressant effects of bright light.