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updated 1/27/2011 2:49:28 PM ET 2011-01-27T19:49:28

Winter, for those who live far from the equator, means cooler temperatures, fewer daylight hours and occasional pangs of "winter blues." While these feelings of mild malaise come and go easily for some, each winter signifies a new, unshakable cycle of depression for Rebecca Davis that is more serious.

"I had grown up in Minnesota and North Dakota and went to boarding school in Michigan. It was there that I realized what a profound effect the weather has on my mood, personality and all aspects of my life," said Davis, a publicist who now splits her time between New York City and St. Petersburg, Fla.

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"My doctor determined it was seasonal affective disorder after my time in Michigan. He put two and two together, and noticed my symptoms are far worse when clouds are in the sky," Davis said.

About 1 to 10 percent of adults in the United States suffer from seasonal affective disorder (SAD). First described as a medical condition in 1984, SAD is now listed by the Diagnostic and Statistical Manual of Mental Disorders as a type of major depression.

Symptoms of SAD
However, unlike conventional depression, SAD is unique in that it always recurs at a particular time of the year. And although SAD is frequently thought of as a winter depression, it can affect patients during other seasons, too.

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The cause of SAD is unknown, but it has been hypothesized that the lack of sunlight disrupts the body's circadian rhythm.  According to research published in the journal Nature by Johns Hopkins University neuroscientists, our eyes have special photoreceptors that monitor light levels. These could be linked to the "non-visual" ways we respond to light, such as setting the body’s circadian pacemaker and affecting mood and temperament.

Changes in light levels could explain why the majority of SAD patients, including Davis, experience symptoms during winter and remission during spring.

"I get the classic symptoms of depression when the weather is gray, like feelings of low self-esteem, anxiety, obsessive worry and just a general inability to be joyful about anything," Davis said.

Aside from these symptoms, some SAD patients go through bouts of craving carbohydrates. They are also likely to be more sleepy. Based on what they report, SAD patients average an additional 2.5 hours of sleep a day during winter than during the rest of the year; the general population sleeps about 0.7 hours more in the winter.

How to cope
While some SAD sufferers may be too lethargic or depressed to leave the house, outdoor activities and regular exercises can help alleviate the symptoms. A 2007 study by Duke University researchers confirmed that exercise is comparable to antidepressant medication in treating patients with major depressive disorders.

And because the illness is linked to the production of serotonin in the brain and the duration of exposure to bright sunlight, according to a 2002 Lancet study, increasing your exposure to daylight can be comforting.

"I felt the difference immediately after I moved from New York to Florida. Actually this is pretty much the case as soon as the sun comes out, regardless of where I lived," Davis said.

When moving to a sunnier locale isn't an option, a light box can help.

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There are no official "dosage" guidelines for light therapy. However, most physicians recommend using a specialized light box that mimics outdoor light by providing adequate brightness, proper wavelength of light and appropriate ultraviolet light filtration.

A 2009 study at Boston's Brigham and Women's Hospital suggested that SAD patients respond more favorably to blue-hued light rather than broad-spectrum, white light. However, the researchers said more studies are needed to determine the optimal therapeutic brightness and color.

In any case, users should make sure their light box was designed for SAD sufferers. According to the Mayo Clinic, light boxes designed for skin therapy may emit too much harmful ultraviolet light to the eyes. Similarly, tanning beds are not proper substitutes.

The brightness recommendations given in the past have ranged from 6 hours per day of 2,500 lux (an amount of light approximately equivalent to the brightness of five 100-watt incandescent bulbs) to 1 to 2 hours a day at 1,500 to 2,500 lux.

Davis has found the light box helps her. "I use it anytime when it's likely to be cloudy a few days in a row," she said.

Like people with other mental illnesses, SAD patients can benefit greatly from cognitive behavioral therapy. However, unlike conventional therapy, the CBT sessions are likely to focus more on the seasonal aspect of mood changes, said Yael Nillni, a doctoral candidate at the University of Vermont who is involved in ongoing SAD research. Some preliminary data have shown that talk therapy could work better than light therapy in the long run.

"At first, light therapy and CBT both do an equally good job of correcting mood disorders," she said. "However, we start to see a difference during the 1-year and 2-year follow-up. We see that CBT does a better job of prepping the patient and preventing future occurrences of depression during the following winter."

The results can vary among patients, however. Although Davis tried out CBT in her early 20s and found it helpful at times, she said she has relied mainly on antidepressants to control her symptoms.

"SAD is about general mood, and I just don't think it's possible to talk yourself out of feeling blue," Davis said.

SAD can be a long-term illness, but it doesn't have to be debilitating. Davis has been coping with it for 15 years and has kept her symptoms in check with antidepressants and exercise. She is also adapting very well to sunny Florida.

"I think most of my advice would be intuitive to anyone who craves sunlight. Insist on a workspace with natural light. Same goes for finding a home with ample windows and good sunlight. Throw back the blinds and shades on sunny days, even if it is cold," she said.

"And during the day, exercise, exercise, exercise!" she said. "Nothing gives me more energy on dreary days."

Pass it on: Antidepressants, therapy, exercise and light-box treatments may help alleviate seasonal affective disorder.

Follow MyHealthNewsDaily on Twitter @ MyHealth_MHND.

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