DEAR DR. ROACH: Would you write about seasonal affective disorder to help so many others who may be suffering with this, but don’t even know it? I was diagnosed with clinical depression following a move from Southern California to Central Wisconsin. I took medication and had counseling, and after a couple of years was doing well. My doctor and I decided I could go off Prozac.

I did really well for a while, but then in the winter, I became depressed again. This time, the antidepressants did not help at all. In the spring, I read an article describing seasonal affective disorder, and recognized myself! I was so excited to learn more that I contacted my psychiatrist, who said he didn’t really know much about it, but gave me a small article from a professional journal that included information about light therapy.

I bought a light box and have used it every year since then. Now I do not need antidepressants at all! I believe that this is something many people are not aware of, and with the long winter nights coming on soon, it might be something that you would write about. Since I got my SAD lamp, the availability of high-quality therapeutic lights has definitely grown. I truly believe that light therapy saved my life. It has made the winters bearable. Every time I hear someone say, “I HATE winter,” I ask if he or she has ever heard of SAD, and talk about my experience. — T.M.H.

ANSWER: Seasonal affective disorder is a condition of recurrent mood disorders, especially major depressive illness episodes, occurring in a particular season, usually winter. Without treatment, symptoms often improve in spring or early summer. It is far more common than you might think — 0.5 percent to 3 percent of the population met criteria for this disorder in a well-done study. Worse, 60 percent of people diagnosed with SAD have never been offered treatment for the condition.

What causes SAD is unknown. Theories include issues with circadian rhythms, relating to the decreasing amount of sunlight; changes in melatonin synthesis; and changes in the sensitivity of the retina to light.

Treatment for SAD may consist of light therapy, which worked well for you, but does not work as well for others; drug therapy, such as fluoxetine (Prozac) or bupropion (Wellbutrin); and talk therapy. These may be more effective when used in combination. In addition to these primary therapies, careful attention to sleep and exercise are useful adjunct treatments.

Light therapy can be done many different ways. One well-studied method is bright light (10,000 lux, a measure of brightness) for a half-hour each day beginning 10 minutes after awakening. Other studies have shown good results with less-intense light (2,500 lux) for longer periods (two hours). Finally, simulating dawn with gradually increasing light has been effective in several trials. As always, finding an experienced mental health professional is likely to result in better outcomes. SAD is sufficiently well-known now, at least by most mental health specialists, so finding an expert will be much easier than it was years ago.

A few people have the opposite pattern of SAD, having depression or mania during the summer. Light therapy is not effective in these cases. In fact, limiting sunlight during the day and keeping cool at night may help.

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Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu or send mail to 628 Virginia Dr., Orlando, FL 32803.

Dr. Roach