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Evidence That the Therapeutic Effects of Light Are Mediated by Circadian Phase Shifts

Robert L. Sack, MD; Alfred J. Lewy, MD, PhD; David M. White, PhD; Clifford M. Singer, MD; Marion J. Fireman, MD; Robert Vandiver, MD

Arch Gen Psychiatry. 1990;47(4):343-351.

Abstract
• Bright light exposure has been found to alleviate the symptoms of recurrent winter depression in many patients. The mechanism of light therapy may involve shifts in the timing (phase) of circadian rhythms. In this study, morning light exposure (which shifts rhythms earlier) was compared with evening light exposure (which shifts rhythms later) in a double-blind, crossover design. The onset of melatonin secretion in the evening was measured under dim light conditions as a marker for circadian timing (phase) before and after each treatment. Eight patients with winter depression and five control subjects were studied. Morning light was found to be significantly better than evening light in reducing depressive symptoms. At baseline, there was a trend for the onset of melatonin production to be later in the patients than in the controls. Morning light shifted the melatonin onset significantly earlier in the patients but not the controls. Our findings suggest that patients with winter depression have circadian rhythms that are abnormally delayed and that bright light therapy benefits winter depression by providing a corrective advance.

Author Affiliations
From the Sleep and Mood Disorders Laboratory, Department of Psychiatry, School of Medicine, Oregon Health Sciences University, Portland (Drs Sack, Lewy, Singer, Fireman, and Vandiver), and the Department of Psychiatry and Behavioral Sciences, University of Washington School of Medicine, Seattle (Dr White).

Footnotes
Accepted for publication June 30, 1989.

Reprint requests to Mail Code L-469, Oregon Health Sciences University, Portland, OR 97201 (Dr Sack).

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