Here's the truth about seasonal depression
SAD has been a diagnosable variation of depression in the Diagnostic and Statistical Manual of Mental Disorders (DSM) since the 1980s.
Patients often report they have SAD, but there wasn't very much hard data on how many people were actually affected.
So researchers from Auburn University at Montgomery analyzed a national survey of over 34,000 US adults to see if they could find evidence of SAD based on people's symptoms of depression, their locations, and the seasons. They published their results in January 2016 in the journal Clinical Psychological Science.
The researchers found that reported depression was not linked to season, latitude, or sunlight exposure, and concluded that there seemed to be more evidence for SAD in "folk theory" than in scientific research.
"The findings cast serious doubt on major depression with seasonal variation as a legitimate psychiatric disorder," they wrote, adding that while the study "cannot rule out the possibility that major depression with seasonal variation exists," it suggests that - if it does exist - it is extremely rare. (They admit that their survey could have missed a small subset of the population that does have SAD.)
The UK's National Health Service (NHS) pointed out that the survey-based study had some significant limitations. "Depression was not a clinical diagnosis - it was based on the participant's response to a questionnaire over the phone," the NHS site pointed out. "This presents problems, as some people may not have answered questions truthfully, and those with depression or SAD may not have answered the phone."
But even if some people do suffer from SAD, the study authors emphasized that we shouldn't confuse cause and effect with chance.
"Merely being depressed during winter is not evidence that one is depressed because of winter," they wrote.