Now that the days are shorter and colder, it's time to talk about seasonal affective disorder (SAD). A new video from the American Chemical Society's "Reactions" series sheds some light, as it were, on the topic. Sure, many of us aren't too excited about the challenges that winter might bring. Walking through slush and feeling anxious about holiday parties probably aren't ideal conditions for a lot of people. But, depending on where they live, anywhere from 1% to 9.7% of Americans experience SAD — a.k.a. "depressive disorder with seasonal pattern" — and these people feel even worse about seasonal changes. As you may have guessed, the condition is related to depression. But, unlike major depression, symptoms tend to ebb and flow with the changing of the seasons, usually intensifying around fall and winter. Researchers believe this may have to do with light cues from the sun and the way our bodies react to them. With less light in the colder months, our hormonal cycles get out of whack, and for people with SAD, this could mean big changes in mood. So get cozy, and check out the full video above to learn more about the condition, along with the most common ways to treat it.