Light therapy, long used to help improve mood in people who get the blues when days are darker and shorter, can also treat non-seasonal depression, a small Canadian study suggests.

Depression is one of the most common mental health disorders and a leading cause of disability and reduced quality of life worldwide, the researchers note in JAMA Psychiatry.

While many people with depression may be helped by medication or psychotherapy, these options don't work for everybody and some patients don't stick with treatment due to side effects or challenges related to access or affordability.

"This study is the first to show that light therapy alone is effective versus a placebo, and the first to compare combination light and drug to light alone," said lead study author Dr. Raymond Lam of the University of British Columbia.

"The combination of light therapy and antidepressant was the most effective," Lam added by email. "However, some people may prefer to try a non-medication treatment first, and may elect to start with light therapy."

To test the effectiveness of light therapy for non-seasonal depression, Lam and colleagues randomly assigned 122 patients to one of four groups for eight weeks: only medication, only light, a combination of drugs and light therapy, or a control group that received no active treatments.

The active treatments in the study included a daily 20-milligram dose of fluoxetine (Prozac) and daily exposure to a fluorescent light box for 30 minutes after waking up each morning.

Researchers gave people assigned only light therapy a placebo, or inactive pill, and they provided participants assigned only to drug treatment with an inactive device instead of a working light box. The control group received both the placebo pill and the inactive device.

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At the start of the study, researchers used a standard questionnaire to assess the severity of depression by asking participants about sadness, inner tension, reduced sleep, reduced appetite, concentration difficulties, lack of energy, inability to feel, and pessimistic or suicidal thoughts.

The highest score is 60, indicating the most severe depression. On average, participants had scores of about 26 or 27 at the start of the study, pointing to moderate depression.

After eight weeks, the group receiving both medication and light therapy had the biggest average drop in depression scores, with a 16.9 point decline.

People who got only light therapy had an average 13.4 point decline in depression scores, while people who had only medication experienced an 8.8 point drop and the control group getting only inactive treatments had a 6.5 point decrease.

While the exact reason light therapy might ease depression is unknown, it may have worked by helping to reset the biological clock in the brain, or circadian rhythms, the researchers hypothesized.

One shortcoming of the study is that researchers lacked data on how much exposure people had to natural light during the study, the authors acknowledge. It's also possible that a higher dose of medication or light therapy might have made a bigger impact.

Normally, if an antidepressant doesn't work in a low dose, clinicians will gradually increase the dose to help achieve the right amount of medicine to ease symptoms, noted Dr. Teodor Postolache, a psychiatry researcher at the University of Maryland School of Medicine in Baltimore who wasn't involved in the study.

"Light treatment could be an adequate agent that in combination with medications may make the response to treatment more complete," Postolache said by email. "However, I would emphasize that patients should not buy a light treatment device and start adding it to their antidepressants on their own."

The best time in the course of treatment to consider light therapy may also depend on the severity of symptoms, said Sue Penckofer, a researcher at Loyola University in Chicago who wasn't involved in the study.

"If somebody has more mild depression they might try light therapy first, but if they have more moderate or serious depression medication might be recommended instead," Penckofer said.