But many mental health clinicians have suspected that, instead, light therapy's reported effects reflect depressed volunteers' great expectations about the power of indoor illumination.
Two new studies, both published in the October Archives of General Psychiatry, counter this clinical skepticism with demonstrations that light therapy, especially if administered in the morning, dampens winter depression much more than placebo treatments do.
Biological mechanisms through which bright lights produce antidepressant relief remain poorly understood. A third study in the same issue suggests that morning doses of bright light work by moving up by an hour or two the nightly secretion of melatonin, a hormone involved in sleep regulation and daily biochemical rhythms.
"Light therapy should be considered a mainstream antidepressant [therapy]," remarks psychologist Anna Wirz-Justice of the Psychiatric University Clinic in Basel, Switzerland, in an accompanying comment. "Light is as effective as [antidepressant] drugs, perhaps more so."
Along with feelings of sadness, anxiety, and lethargy, winter depression also frequently includes difficulty awakening in the morning, daytime drowsiness, cravings for sweet or starchy foods, and big weight gains. An estimated 1 in 10 people in Alaska and other northern regions-and 1 in 100 people in Florida-experience this condition, which is also known as seasonal affective disorder.
In one of the new studies, a team led by psychologist Michael Terman of Columbia University recruited 124 volunteers, ages 18 to 65, who exhibited SAD. Over 20 to 28 days, 85 participants received daily 30-minute exposures to bright light from a box mounted above the head. Some had light therapy in the morning, others in the evening, and some switched from one time to the other halfway through the trial.
The remaining volunteers sat for 30 minutes each morning in front of an apparatus called a negative-ion generator, which emitted either low or high densities of air ions. These treatments were intended to serve as placebos.
About 60 percent of those who received morning light therapy displayed marked improvement in SAD symptoms, compared with about 30 percent of those on the evening light regimen. Winter depression eased in only 5 percent of volunteers exposed to low-density ions.
Lessening of SAD also occurred in 40 percent of those exposed to high-density ions, a finding that needs to be explored further, the scientists say.
The second study, directed by psychologist Charmane I. Eastman of Rush-PresbyterianSt. Luke's Medical Center in Chicago, found that nearly 60 percent of volunteers diagnosed with winter depression greatly improved after fou weeks of daily sessions of morning bright light lasting 90 minutes.
The proportion of treatment responders reached only 44 percent for those given evening bright-light doses and 36 percent for volunteers whose daily regimen consisted of sitting in front of a negative-ion generator that made a soft hissing sound but, unknown to them, was not turned on.
Eastman's group studied a total of 96 people diagnosed with SAD.