As many as 20 percent of Americans suffer from seasonal affective disorder, the condition commonly referred to as the winter blues. But for so many sufferers, seasonal affective disorder, or SAD, is much more than a simple case of the blues. SAD brings with it symptoms of depression that can vary from mild to debilitating. But what has puzzled doctors for years is that these symptoms only occur during the fall and winter months.

Shortened days and lower exposure to sunlight have often been blamed as the cause for SAD, but why are some people more prone to this conditions than others?

Brain scans provide the answer. Researchers at the University of Copenhagen in Denmark discovered that during the winter months, people with SAD had higher levels of the transporter protein that removes serotonin, the compound often associated with happiness. This difference means there is less serotonin left in the brain and thus a diminished feeling of happiness.

"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," said lead researcher Brenda McMahon.

Researchers found that people who are not affected by SAD do not have this increase in SERT activity in the winter months. Thus, their serotonin levels are kept relatively stable throughout the year.

This new research may help scientists understand the cause of SAD, but it doesn't necessarily help the sufferers. No matter the cause, following a healthy diet and getting plenty of fresh air and exercise are still the best ways to combat this seasonal condition. Light therapy is another good option for increasing serotonin levels in the brain.

Talk to your health care provider if you think you might be suffering from SAD.

Why seasonal affective disorder makes you sad
Brain scans help doctors understand the underlying cause.