[{"document_url": "https://www.huffingtonpost.ca/2014/10/28/seasonal-affective-disorder_n_6063684.html", "unformatted_src": "https://o.aolcdn.com/images/dims3/GLOB/legacy_thumbnail/630x315/format/jpg/quality/85/http%3A%2F%2Fi.huffpost.com%2Fgen%2F2218702%2Fimages%2Fn-SEASONAL-AFFECTIVE-DISORDER-628x314.jpg", "src": "https://o.aolcdn.com/images/dims3/GLOB/legacy_thumbnail/630x315/format/jpg/quality/85/http%3A%2F%2Fi.huffpost.com%2Fgen%2F2218702%2Fimages%2Fn-SEASONAL-AFFECTIVE-DISORDER-628x314.jpg", "formatted_filename": "http  A F Fi"}, null, null, {"document_url": "https://www.huffingtonpost.ca/2014/10/28/seasonal-affective-disorder_n_6063684.html", "unformatted_src": "https://o.aolcdn.com/images/dims3/GLOB/legacy_thumbnail/150x150/format/jpg/quality/85/http%3A%2F%2Fi.huffpost.com%2Fgen%2F2375034%2Fimages%2Fn-ELF-ON-THE-SHELF-IDEAS-628x314.jpg", "src": "https://o.aolcdn.com/images/dims3/GLOB/legacy_thumbnail/150x150/format/jpg/quality/85/http%3A%2F%2Fi.huffpost.com%2Fgen%2F2375034%2Fimages%2Fn-ELF-ON-THE-SHELF-IDEAS-628x314.jpg", "formatted_filename": "http  A F Fi", "alt_text": "Naughty Elf On The Shelf Photos Will Make You"}, null, null, {"document_url": "https://www.huffingtonpost.ca/2014/10/28/seasonal-affective-disorder_n_6063684.html", "unformatted_src": "https://o.aolcdn.com/images/dims3/GLOB/legacy_thumbnail/150x150/format/jpg/quality/85/http%3A%2F%2Fi.huffpost.com%2Fgen%2F2354848%2Fimages%2Fn-BOYFRIEND-GIFT-628x314.jpg", "src": "https://o.aolcdn.com/images/dims3/GLOB/legacy_thumbnail/150x150/format/jpg/quality/85/http%3A%2F%2Fi.huffpost.com%2Fgen%2F2354848%2Fimages%2Fn-BOYFRIEND-GIFT-628x314.jpg", "formatted_filename": "http  A F Fi", "alt_text": "20 Christmas Gifts Your Boyfriend Will"}, null, null, {"document_url": "https://www.huffingtonpost.ca/2014/10/28/seasonal-affective-disorder_n_6063684.html", "unformatted_src": "https://o.aolcdn.com/images/dims3/GLOB/legacy_thumbnail/150x150/format/jpg/quality/85/http%3A%2F%2Fi.huffpost.com%2Fgen%2F2371292%2Fimages%2Fn-SECRET-SANTA-GIFT-IDEAS-628x314.jpg", "src": "https://o.aolcdn.com/images/dims3/GLOB/legacy_thumbnail/150x150/format/jpg/quality/85/http%3A%2F%2Fi.huffpost.com%2Fgen%2F2371292%2Fimages%2Fn-SECRET-SANTA-GIFT-IDEAS-628x314.jpg", "formatted_filename": "http  A F Fi", "alt_text": "30 Last-Minute Secret Santa Gift Ideas Under"}, null, null, null, null, null, null, null, null, null, null, null, null]
Marisa via Getty Images

The days are getting darker, the weather is chillier, and you can't help but feel a little sad about fully heading into winter. But you should check to make sure you're not dealing with SAD — that is, seasonal affective disorder, a type of clinical depression that follows a seasonal pattern, and often affects people between the periods of fall and winter.

But SAD isn't just about feeling bummed out about the cold or lack of sunlight. It's a mental illness that can cause changes in your appetite, weight, and sleeping patterns. And although science hasn't been able to pinpoint the exact cause of it, new research from the University of Copenhagen suggests the chemicals in our brain have something to do with it.

"We believe that we have found the dial the brain turns when it has to adjust serotonin to the changing seasons," lead researcher Brenda McMahon said in a statement.

As the BBC explains, the brains of people with SAD are not able to control serotonin (the neurotransmitter that helps create happy moods). In colder months, the serotonin transporter (SERT) protein, which halts the action of serotonin, tends to increase. In short, during cold, dark months, your brain doesn't receive as many happy signals as it does in warmer months.

The small study compared 11 people with SAD to 23 volunteers who didn't have the disorder between summer and winter months. Using intensive brain scans, researchers concluded SAD patients showed five per cent higher levels of SERT, compared to the other volunteers' levels during winter.

"The serotonin transporter SERT carries serotonin back into the nerve cells where it is not active — so the higher the SERT activity, the lower the activity of serotonin," McMahon adds.

And as we head into setting our clocks back this weekend, it may also be a good time to evaluate your current mood. Not sure if you have SAD? Check out some of these common symptoms and treatments:

Photo gallery10 Facts About Seasonal Affective Disorder See Gallery Seasonal Affective Disorder: Study Discovers Biochemical Cause
1/ 10
10 Facts About Seasonal Affective Disorder
MORE: feeling sad Living quick study sad disorder sad seasonal affective disorder Seasonal Affective Disorder seasonal affective disorder symptoms seasonal affective disorder treatment seasonal affective disorder winter seasonal depression seasonal depression disorder seasonal light disorder seasonal mood disorder video what is sad what is seasonal affective disorder winter blues winter depression