The precise root cause of seasonal depression has eluded scientists for years. Now researchers think they’ve found the answer: daylight saving time. They published their report in the journal Epidemiology.

Seasonal depression or seasonal affective disorder (SAD) affects around 1.6 billion people across the globe. Its symptoms mirror those of generalized depression; what differentiates SAD is the timing of its onset, which coincides with winter’s shorter days and long, dark nights.

We know that sunlight, or the absence of it, has a powerful effect on our bodies. But scientists have yet to find a definitive physiological link between darkness and SAD, a fact that makes some wonder if there aren’t other variables at play.

Previous studies have found a relationship between the shift from daylight saving into standard time and other health problems, but they had not looked specifically at the transition’s effect on depression. To get a better idea, an international team of researchers looked at Danish hospital intake records from 1995 to 2012, including 185,419 diagnoses of depression.

As expected, they saw an increase in hospital admissions for depression as winter descended. But that increase spiked at one particular time: the month immediately following the changing of clocks.

The researchers controlled for variables like day length and weather, which they say confirms that the 8 percent rise in depression diagnoses was not a coincidence.

And while their study focused on people with severe depression, the authors say the time shift likely affects “the entire spectrum of severity."

Though the study did not identify the mechanism responsible for time change–related depression, the researchers believe it may have something to do with the way daylight saving manipulates our hours of light and dark. Danish daylight saving protocol steals an hour of daylight from the afternoon and moves it to the early morning—a time, the authors say, when most people are indoors anyway.

"We probably benefit less from the daylight in the morning between seven and eight, because many of us are either in the shower, eating breakfast or sitting in a car or bus on the way to work or school. When we get home and have spare time in the afternoon, it is already dark," co-author Søren D. Østergaard of Aarhus University Hospital said in a statement.

Then there are the psychological effects. In changing the clocks, we are forced to acknowledge the arrival of months of darkness, a realization that Østergaard says “is likely to be associated with a negative psychological effect.”

Fortunately, while we still don’t fully understand the causes of SAD, we have found effective treatments. If you find yourself depressed as the year winds down, talk to your doctor and look into a therapeutic light box.

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