The findings from a new study lend further support to the “circadian phase-shift hypothesis” as the basis for winter depression, also known as seasonal affective disorder (SAD).

According to the phase-shift hypothesis, the authors explain, SAD is caused, at least in part, from a mismatch between circadian rhythms related to the sleep/wake cycle and rhythms more closely associated with the body’s own “circadian pacemaker.”

For most patients with SAD, depression occurs in winter because of the late sunrise, “causing their circadian rhythms to delay with respect to clock time and with respect to the sleep/wake cycle.”

As reported in the Proceedings of the National Academy of Sciences, Dr. Alfred J. Lewy, from Oregon Health and Science University in Portland, tested the hypothesis by giving low-dose melatonin or placebo to 68 patients with SAD. Melatonin is a “chemical signal of darkness,” and an ideal agent for inducing phase shifts.

If the phase-shift hypothesis were correct, giving melatonin in the afternoon to the typical SAD patient would cause a shift in their wake cycle to earlier in the day, resulting in an improved mood. By contrast, giving melatonin or placebo in the morning would not be expected to improve mood.

The authors found that a low dose of melatonin could be given safely to the subjects and did not cause sleepiness. As anticipated, in typical SAD patients, treatment with melatonin in the afternoon worked best at alleviating their depression.

Conversely, in the smaller group of patients with phase-advanced disease (possibly triggered by the earlier winter dusk), treatment in the morning appeared to be optimal.

“The findings support the phase-shift hypothesis for SAD, as well as suggest a way to assess the circadian component of other psychiatric, sleep, and (time-related) disorders,” the authors conclude.

SOURCE: Proceedings of the National Academy of Sciences, April 24th early edition, 2006.