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