Light Therapy in Seasonal Affective Disorder Is Independent of Time of Day or Circadian Phase

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We tested the hypothesis that phase-delayed circadian rhythms underlie seasonal affective disorder (SAD) by measuring phase position of 6-sulfatoxymelatonin excretion and comparing antidepressant response to morning or evening light given as a first treatment.


Randomized controlled trial.




Thirty-two women and seven men with SAD.


Light therapy (2500 lux for 1 hour for 1 week) was administered either at 7 am or 10 pm, preceded by a baseline week and followed by a withdrawal week.


Our SAD patient sample was moderately depressed (Hamilton Depression Scale (HAM-D) score 18); a HAM-D reduction of 50% or more was found in 12 of 18 patients given morning and in 15 of 21 patients given evening light (70% response rate). Response was not dependent on age, gender, stage of the menstrual cycle, time of year, or on the timing or duration of sleep. Urinary 6-sulfatoxymelatonin was measured in 30 patients; 22 had phase-delayed circadian rhythms. However, phase position was correlated neither with depth of depression nor with a preferential response to morning or evening light.


Both morning and evening light therapy improved depressive symptoms in patients with SAD independent of their circadian phase or sleep timing. These findings argue against a circadian phase-delay hypothesis of the pathophysiology of SAD, or the necessity of a phase-advance by morning light for clinical efficacy. They additionally suggest more practicable and flexible schedules for light therapy in SAD, since time of day is not crucial.

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