The study was led by Alfred Lewy, M.D., Ph.D., an internationally recognized pioneer in the study of circadian (24-hour) rhythm disturbances, such as those found in air travelers and shift workers, as well as in totally blind people.
Lewy and his colleagues in the OHSU Sleep and Mood Disorders Lab set out to test the hypothesis that circadian physiological rhythms become misaligned with the sleep/wake cycle during the short days of winter, causing some people to become depressed. Usually these rhythms track to the later dawn in winter, resulting in a circadian phase delay with respect to sleep similar to what happens flying westward. Some people appear to be tracking to the earlier dusk of winter, causing a similar amount of misalignment but in the phase-advance direction. Symptom severity in SAD patients correlated with the misalignment in either direction.
The treatment of choice for most SAD patients is bright light exposure, which causes phase advances when scheduled in the morning. Because patients know when they are exposed to bright light, however, there is a considerable placebo response associated with it. Melatonin can also cause phase advances, but it has to be taken in the afternoon. The Lewy team used afternoon melatonin to test if it was more antidepressant than melatonin taken in the morning, which causes phase delays.
The researchers randomly assigned 68 SAD patients to one of three treatment groups, taking placebo capsules or melatonin in the morning or afternoon for three weeks. After four years of study, they concluded that, similar to persistent jet lag, circadian misalignment is a major part of SAD.
Source:Oregon Health & Science University