"On the West Coast where we live it's so rampant, I can't even tell you how many people have it," said Kane. "Everyone is feeling it with the gray skies and rain. It's like nighttime all the time here."
Doctors there routinely prescribe fish oil and vitamin D, as well as light therapy to balance out the sleep hormone melatonin and "boost" the feel-good hormone serotonin, according to Kane. Many are also on antidepressants.
"A lot of people depend on alcohol and drugs all of a sudden," she said. "They are stuffing themselves with carbs and their food intake is up. They have depression symptoms -- what's the point of getting out of bed in the morning when they feel no energy and there is dark all over them?"
"Some suffer so bad, they can't function," said Kane. "Everyone here who can afford to get away for two weeks in the winter, go to Hawaii."
Even those who live south of the Mason-Dixon Line in the United States can be affected.
Tina Saratsiotis, who works for a faith-based nonprofit group in Towson, Md., was surprised to develop SAD several years ago.
"I used to be a night person and like the dark. Then something changed," she said. "By fall when it gets darker and the fatigue and sadness comes and by Christmas, it's difficult to function."
"It creeps in slowly -- I eat more and have trouble concentrating," she said. "I am more irritable and weeping, like a prolonged version of PMS. It makes it hard to get things done and to enjoy things."
Columbia's Terman said there may be genetic influences in who gets SAD -- a vulnerability to depression and to insufficient light exposure.
SAD sufferers say it's especially hard on their relationships when their winter moods kick in.
"Now, he's very understanding," said Saratsiotis, who uses both light therapy and antidepressants to deal with the condition. "But before, when I didn't feel up to going out, I couldn't explain not feeling great. People wonder, 'Why doesn't she like me?' and, 'She's no fun.'"
But when spring rolls around, so does her old self.
"I love the solstice -- thank you, Lord, for the solstice," she said. "I really need [the medication] now, but I may not in the spring and summer."
But now, in when the days are their shortest, SAD puts a crimp on the holidays.
"It kills Christmas," said Saratsiotis. "I sit in the middle of the department store with that particular song about the sleigh bells ringing, and I am sobbing. I burst into tears and think, 'Just kill that song.'"