With the days getting shorter, and sunlight becoming more precious, millions of Americans will experience seasonal affective disorder (SAD). SAD is actual depression that afflicts sufferers again and again during the fall and winter months (although others experience recurrent depression during the warmer seasons, instead). Symptoms typically include a decrease in mood, anxiety, low energy, trouble concentrating, an increased need for sleep, appetite changes (usually increased, including craving carbohydrates), decreased sexual feelings, hopelessness and a lack of interest in activities that the person used to enjoy. Like other forms of major depression, SAD can also trigger thoughts of suicide.

The cause of SAD may relate to levels of two chemicals in the brain - melatonin and serotonin. Excessive melatonin levels have been linked to depression, and melatonin levels in the brain tend to rise during the colder months. Serotonin levels, conversely, fall during the winter, when exposure to sunlight is decreased.

Those who have experienced some of the symptoms listed above with the change of seasons last year should think about whether SAD is affecting you now or if it will affect you in the coming weeks and months. If family members of yours have a similar pattern, that's all the more reason to wonder whether SAD is responsible for what you're feeling.

SAD, like every form of depression, is highly treatable. Psychotherapy and medication are very effective. But there's a special treatment for SAD that can help in as many as 80 percent of cases_ light therapy. And trying light therapy first makes good sense, provided you aren't experiencing severe symptoms, especially thoughts of harming yourself.

Many companies (including ETA Lighting Systems, Northern Light Technologies and LiteBook) sell light therapy devices that contain fluorescent or LED bulbs that mimic sunlight. The bulbs are housed behind a plastic screen and don't expose users to any harmful ultraviolet rays. Sitting in front of a light box for as little as 30 minutes a day (although longer periods are required in some cases) can provide dramatic relief.

Clinical trials haven't yet convinced the Food and Drug Administration to approve bright light therapy as a treatment for SAD, but many scientists are convinced of its benefit. Some clinical trials find it as effective as Prozac, with quicker results and many fewer side effects. In my own practice, I've found it tremendously helpful for many patients over the years.

The first key to defeating SAD is recognizing it. Too many people suffer needlessly, thinking they just hate the fall or winter, or get the blues when they can't get outside enough. But if you dread crisp air, falling leaves and the thought of snow on the ground, it's worth wondering how intense that dread really is, and whether symptoms of SAD lurk behind it.

Dr. Keith Ablow is a psychiatry correspondent for FOX News Channel and a New York Times bestselling author. His newest book, "Living the Truth: Transform Your Life through the Power of Insight and Honesty" has launched a new self-help movement. Check out Dr. Ablow's website at

livingthetruth.com

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Dr. Keith Ablow is a psychiatrist and member of the Fox News Medical A-Team.