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It wouldn't be winter in Canada without unusual weather patterns -- extra mild weather in parts of the country, heavy snowfall in others. But one thing remains constant at this time of year: About 5 per cent to 10 per cent of our population will experience seasonal affective disorder, commonly known as SAD.

SAD symptoms include changes in mood, energy and appetite, excessive sleepiness, fatigue, weight gain and craving for carbohydrates, loss of libido and depression. The disorder's hallmark feature is that symptoms are present in darker fall and winter months, but gone in spring and summer.

It is well known that seasonal affective disorder increases, the farther north you go. A study of U.S. soldiers in Alaska, for example, found about 13 per cent experienced the condition, much higher than the general population rate. And it has been noted that people from southern latitudes are at increased risk of developing the disorder if they move north.

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It isn't known for sure what causes seasonal affective disorder, but the "latitude theory" suggests that lower exposure to sunlight increases the risk. There's no doubt people feel down in the dumps on dark, gloomy days and much more energized when they get a shot of sunlight.

Thus, many people believe that light is vital to fighting off the disorder. But what about blind people, who cannot distinguish between dark and light; wouldn't they experience a higher rate of the condition?

To explore this question, we have to take a look at our eyeballs and the role they play in our body rhythms. At the back of our retina are receptors, the so-called rods and cones that are responsible for interpreting what we see and are essential to our vision.

But researchers at Harvard University have recently discovered that there is another kind of receptor in our eyeball -- a non-rod, non-cone receptor essential for recognizing the presence of light.

This third receptor is critical to the workings of our circadian rhythm, because darkness triggers the synthesis of melatonin, often called the "sleep hormone." Daylight, in turn, suppresses the production of melatonin.

In blind people who do not have rod or cone functions, but do have the receptors to recognize light, the circadian rhythm remains normal and the incidence of SAD is about the same as the general population. But in people who do not have eyeballs at all, and thus don't have the third receptor, the circadian rhythm is altered dramatically. Their melatonin production is distorted. They are in a constant state of jet lag, if you will. (So far, no research has been done on whether this group has a higher rate of seasonal affective disorder than other blind people, or the general population.)

Along with the role of light in the condition, there may be a genetic component that makes some people more susceptible to it than others. Studies show that genetic adaptations may have evolved to deal with lower light levels in fall and winter months. For example, direct descendants of Icelandic immigrants in Manitoba have a much lower incidence of SAD, at about 1.2 per cent, than the general population. It's also known that the condition tends to run in families, bolstering the idea that there is a genetic component involved that makes some people more susceptible.

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(Another theory of what causes seasonal affective disorder looks at another brain chemical, serotonin, which your body uses to make melatonin. It is suggested that if serotonin levels are low, melatonin will be low, circadian rhythms will be off kilter and the risk of SAD might increase. Carbohydrates increase serotonin and some researchers believe that the carbohydrate craving seen in seasonal affective disorder might be a body's way of trying to correct serotonin levels.)

The classic treatment for SAD is light therapy, which is about 70-per-cent effective. The idea is to have early-morning exposure to light to turn off melatonin production and ensure the circadian rhythm is normal. While some studies have found that morning light and evening light are equally effective, most research suggests that treatment -- whether outdoors in natural light or using artificial lamps -- is best done in the morning.

Other possible treatment includes taking St. John's wort in combination with light therapy. But this herb can interact with a number of other medications, so be sure to check with your doctor before trying it.

Dr. Marla Shapiro can be seen daily on Balance: Television For Living Well on CTV. Questions about general health issues can be sent to her at: .

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