A long dark winter can be mentally and physically exhausting, but a recent study published in the journal of Clinical Psychological Science challenges the idea that it's making people depressed.

Seasonal affective disorder (SAD) is commonly believed to affect a significant portion of the population in the Northern Hemisphere during the darker winter months.

As many as 35 per cent of Canadians complain of having the "winter blues," according to the Centre for Addiction and Mental Health. Another 10 to 15 per cent have a mild form of seasonal depression, while about two to five per cent of Canadians will have a severe, clinical form of SAD.

The disorder is based on the theory that some depressions occur seasonally in response to reduced sunlight — but recent research says that theory may be unsubstantiated.

No relationship to sunlight

"We conducted a study using data that looked at the relationship between depression in a fairly large sample of people distributed over several degrees latitude in the United States," said Steven G. LoBello, a psychology professor at Auburn University in Montgomery, Ala., and one of the study's authors.

"We looked across the four seasons to see if there was an association with sunlight, and we simply didn't find a direct relationship with sunlight, the seasons, or latitude."

LoBello's study does not look at populations north of the 49th parallel, but he is confident his findings hold.

"We cite in our paper a paper by [Vidje Hansen] that looked at this problem in Norway, which is north of the Arctic Circle, and they experience the polar night."

According to LoBello, that research "did not find any relationship between an increase in depression and the duration of the polar night."

But it's in the DSM

A "seasonal pattern" modifier for depression diagnoses was officially added to the Diagnostic and Statistical Manual of Mental Disorders (DSM) in 1987.

The addition was based on psychiatric research investigating SAD that started to emerge in the early 1980s, but LoBello says it was a hasty addition.

"There was very little research at that time, but yet the DSM authors, I suppose, thought there was enough evidence to attach the diagnostic modifier of seasonal variation to the diagnosis of major depression based on what they found in respect to SAD."

SAD has attracted cultural and research attention for more than 30 years.

Every year, people around the world spend money on products and treatments to try to combat the winter blues, and specialists tout the benefits of things like full spectrum lights and bright light therapy.

I'm not saying people don't get depressed in winter - Prof. Steven LoBello, study co-author

But for LoBello, those ideas are more likely a product of folk psychology, rather than the fruit of objective data.

"Many times in the history of medicine there have been treatments that work, and we don't know why. We may be looking at something like that here, I just don't know."

LoBello says his findings don't mean that people should start tossing out products and treatments they find helpful.

"I think any depression is a serious matter and needs to be treated. If people find relief using bright lights, I'm not going to advocate that they be done away with," said LoBello.

"I'm not saying people don't get depressed in winter. I think the clearest way to understand what we're saying is that I don't believe lack of sunlight is the important causal mechanism there."