About this time of year every year, people begin to feel...off. You might already be starting to get sick of hearing your friends' complaints about how down they are now that we're in the midst of winter. Chances are, at least someone in your circle has gone ahead and diagnosed themselves with seasonal affective disorder (SAD). This disorder has become such a part of our culture that it seems like everyone has it (or at least thinks they have it). But the true story about SAD — which is either the best or worst medical acronym, ever — is more complicated, even controversial, than you might realize.First off, SAD as we know it colloquially isn't actually in the Diagnostic and Statistical Manual (DSM), a.k.a. the giant book that therapists use to make diagnoses. The idea of winter-related sadness has been around for centuries, but was only formally documented in medical research in 1984 as SAD , which was described as a "syndrome characterized by recurrent depressions that occur annually at the same time each year." Today, however, what most experts refer toas SAD is really a form of major depression "with seasonal pattern."As with other forms of depression, the kind that seems to come and go isn't simply feeling down. Clinical depression — with or without a seasonal component — is a serious disorder characterized by loss of interest in things that were once pleasurable, loss of energy, changes in appetite and sleep habits, and, in its most serious forms, suicidal thoughts, explains Dan Oren, MD , who serves on the board of directors for the Center for Environmental Therapeutics . Although anyone with major depression may have recurrent episodes, what we know as SAD is unique in that it seems to consistently return at around the same time every year, primarily in the winter.The current prevailing theory is that a lack of sunlight during shorter winter days is at the root of the disorder, perhaps because it throws off the ebb and flow of our circadian rhythms . That may be because the hormonal changes we go through respond to external cues — including things such as sunlight — and includes the regulation of, among other things, the release of the neurotransmitter serotonin, which is related to mood regulation.Some studies suggest that light-box therapy (possibly in addition to psychotherapy and/or medication) may be a helpful treatment, because it can potentially replace the sun as an external cue for our circadian rhythms. The thinking goes, without getting sunlight every morning, you need something else to keep those biological patterns on track.