There is a reason that when drawing a picture of the sun, people also add sunglasses and a smile. The sunglasses part is arbitrary simply because a sun doesn’t need to be protected from itself, but that’s not the point. The point is that sunlight makes people happy.

Research from United Arab Emirates shows a strong link between positive moods and time spent in sunlight. Researchers have also found that the lack of sunlight in the winter causes people to have an imbalance of serotonin, a brain chemical that affects your mood. Their bodies also make too much melatonin, a hormone that regulates sleep, and not enough vitamin D.

Those are facts. There is, however, a not-so-proven reason that people feel the “winter low”: Seasonal Affective Disorder (SAD). SAD is a type of depression that begins and ends at the same times every year. It usually starts around fall and continues into the winter months. You probably have it if you tend to feel happier in the summer than you do in the winter, continuously. This is a real form of depression that can put a serious damper on winter months.

Despite the facts, there are people who don’t believe that this disorder is real, simply because the gloom of winter tends to be paired with tiredness, sadness and overeating. But the question is, why? Does winter always correlate with these feelings, or is it Seasonal Affective Disorder at work?

BBC News recently tested a theory based on the difference in happiness levels of Midwesterners and Southern California residents. Their test proved two things: one being that people in California are no happier than Midwesterners, and two, out of all the factors that affect life satisfaction, weather was at the bottom.

This doesn’t prove anything. The thing about SAD is that many people brush it off as just the “winter low.” Of course weather is at the bottom— it’s a survey on life satisfaction. People have bigger things to worry about. SAD is a form of depression that people don’t realize even exists. It hides within other feelings and places an overall downward look on life. It makes you look down on the bigger problems, like hating your body in the winter months, unhappiness at work and being reluctant to get out of bed.

Obviously, it’s normal to have bad days. Everyone does. It starts to become a problem when you feel down every day. Overeating during the holidays may not be the only reason you gain weight. The fact that your bed is warm may not be the only reason you can’t seem to get out of bed. Feeling sluggish or hopeless may not just be because Christmas is over. It’s Seasonal Affective Disorder, and it’s exactly why you feel more depressed in winter.