So what exactly causes it? That's not clear. Most researchers agree that a lack of sunlight plays a role, especially since light affects circadian rhythms (that is, your internal clock) and vitamin D levels, among other things. But light is probably not the only cause. The lifestyle changes that come with colder weather, like spending less time exercising outside and more time curled up on the couch, can cause a dip in mood, as can the highs and lows of the holiday season.
While antidepressants can help, there's a drug-free solution that's shown to be just as good for some people: light therapy. This unique treatment entails sitting close to a light box that shines with 10,000 lux—about 100 times brighter than most indoor lighting—for 30 minutes a day, preferably right after waking up. When done at the same time every day, this kind of therapy has been shown to recalibrate your body clock and reduce levels of cortisol, a stress hormone. Learn how light therapy works and find other ways to fight the winter blues in the videos below.