Response of the melatonin cycle to phototherapy for Seasonal Affective Disorder

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Abstract
It is well-established that human nocturnal melatonin secretion is suppressed by presentation of artificial light >2,000 lux, a level that is also therapeutically effective in alleviating winter depression symptoms of Seasonal Affective Disorder [SAD]. Furthermore, early-morning bright light induces phase advances of the melatonin cycle in SAD patients (Lewy et al., 1987 a). The functional significance of melatonin in SAD remains unclear. With plasma melatonin sampled at 20-min intervals in a series of overnight studies, we found marked phase delays of the cycle, relative to that previously reported for normals, in 4/5 depressed SAD patients. 2,500 lux light exposure at 6–8a.m. resulted in exponentially declining melatonin levels that approached low daytime baselines within two hours (t1/2 = 45.52 min). All five patients showed clinical remissions as well as phase advances of the melatonin cycle of 0.75 to 3.27 hours (mean, 1.94± 0.84hours) after one week of daily exposure from 6–8a.m. and p.m. These results suggest that the combination of early morning and early evening light exposures induces circadian phase adjustments similar to those of morning light alone, by impacting a photosensitive interval when, in SAD, melatonin secretion overshoots its normal nocturnal phase.