Editor's note: Dr. Charles Raison, CNNHealth's mental health expert, is an associate professor of psychiatry at the University of Arizona in Tucson.
(CNN) -- I grew up in a place where the sun shone every day from May through October. These sun-drenched days were the happiest times of my life.
But in winter a dense fog would often blanket my hometown for weeks at a time, leaving the world gray and featureless and leaving me down and dreary. These seasons were so definite, so unchanging, that I just came to believe that summer was happy and winter was sad.
No wonder, then, that I've always had a personal fascination with winter depression, known more commonly as seasonal affective disorder, or SAD.
About 15% of the U.S. population experiences the problematic rhythmic change of the seasons. For these individuals, time itself is a stressor, and winter is often a season of despair.
Most of these people who find the season a downer are able to power through the gray days and get by. But about 5% of the population becomes so impaired every winter that they meet the criteria for SAD.
Although everyone is a little different when it comes to how they experience depression, SAD tends to follow a remarkably stereotyped pattern.
Symptoms usually start in the fall as the days shorten. Interestingly, for most people feeling depressed is not what comes first. Rather, people tend to become tired, to sleep more and to eat more -- especially carbohydrates -- and to gain weight as a result.
They have increasing difficulty getting up in the morning and remaining interested in the things around them. Their ability to remember things and to concentrate diminishes. Only as winter bears down in earnest do sadness, despair and other classic emotional symptoms of depression take hold.
Then, as the days lengthen and spring approaches, the symptoms began to evaporate in reverse order, with sadness/despair lifting first and sleep and appetite changes normalizing later.
Many people with SAD experience normal moods from spring through fall, although some will actually become euphoric in spring, a condition known technically as "hypomania." This tendency for SAD sufferers to experience elevated moods during sunny times of the year has led some researchers to see SAD as a variant of bipolar disorder, or manic depression.
Given that SAD is so tied to the seasons and to sunlight in particular, you might predict that people who live in places with cold, dark winters would be more likely than people living in sunny climes to experience SAD. You'd be right.
For example, the rate of SAD in New Hampshire has been estimated at 10%, whereas less than 2% of people living in Florida suffer with the condition. In fact, I've known many patients over the years who only developed SAD when they left places like Florida to live further north.
In this way, SAD is the quintessential environmental disorder -- no winter, no SAD. But that is not the whole story.
Like all psychiatric conditions, SAD occurs at the intersection of genes and environment, of nature and nurture. This point has been brought home by several remarkable studies showing that people in Iceland have remarkably low rates of SAD, despite living in one of the darkest winter environments on earth.
Even more remarkably, people of Icelandic descent living in the prairie provinces of Canada have far lower rates of SAD than their fellow non-Icelandic Canadians, which greatly strengthens the argument that Icelandic people carry an as-yet-undiscovered genetic factor that protects against SAD.
We don't know what causes SAD, although abnormalities in multiple brain regions have been repeatedly observed in studies. The neurotransmitter serotonin seems to be implicated, as does melatonin, one of the hormones most involved in the onset and offset of sleep. Perhaps most intriguingly, several studies suggest that the eyes themselves might contribute to the risk of developing SAD, given that the eyes of people with SAD respond differently to light.
There also seems to be a strong association between being affected by the seasons and having heightened creative abilities.
I was a practicing psychiatrist before I fully admitted to myself that I suffered from mild SAD. In fact, I didn't really believe it until I bought my first light box and turned it on one fall.
The results were amazing. Thirty minutes in front of a bright light (10,000 lux to be exact) and my brain and body felt like they'd been transported back to summer. My mind knew it wasn't true, but that didn't matter; the melancholy was gone.
Of course, my experience is hardly unique, and if you suffer from winter depression you really owe it to yourself to buy a light box and give it a try. You might also look into dawn simulators, which have also been shown to treat SAD.
Studies have also shown that several classes of antidepressants improve SAD symptoms. If one only has depression in the winter, and no other time of the year, antidepressants can be started as a protective measure in the fall before the start of symptoms and stopped again in the spring. Various forms of psychotherapy have also been shown to be effective in treating SAD.
I don't recommend starting any type of medical treatment without first talking with your doctor.
If you are sensitive to winter, there are several other things you can do to help your mood and energy level.
The first seems counterintuitive, and that is to resist the temptation to sleep late or stay in bed on cold, gray mornings.
Even the grayest morning provides many times more light than the typical indoor environment. And light in the morning helps people fall asleep earlier in the evening and to sleep more deeply.
This is important because SAD is tightly associated with something technically known as "phase delay," which means that people with SAD tend to stay up late and sleep late. If you have this behavior, you can improve your mood just by training yourself to move your sleep period forward.
Exercise can also be hugely helpful in combating SAD, especially in the morning. Exercise increases fitness and stimulates the production of antidepressant compounds in the brain and body. Exercising in the morning can also help you fall asleep earlier and sleep better.
I was somewhat unusual in suffering with SAD, because it tends to afflict women more often than men. But I was typical in one fortunate way. SAD is a young person's disease. Many people begin to "outgrow" the condition in their 40s and 50s.
That was the case with me. One winter in my mid-40s the winter blues just didn't happen, and the light box sat unplugged at the back of my desk.
A final note: If you've been thinking to yourself, "I love winters -- summer is what gets me down," let me assure you that you are not alone. Although far less common than winter depression, summer-type SAD is a well-recognized condition. Unlike winter depression, summer depression is all about heat. The hotter it is, the more likely people are to become depressed.
Perhaps because summer depression is so much less common than winter depression, little is known about how to optimally treat it. But I'm put in mind of Mark Twain's comment that the coldest winter he ever spent was a summer in San Francisco, and I suspect that a midsummer journey to a cold, foggy coast might provide tremendous relief.