Every October as the clocks are turned back, Jose Balido notices that his mood changes, almost as if his body were going into hibernation.
His limbs are heavy and he has trouble moving around. Simple household chores like loading the dishwasher seem "insurmountable," he said. But when spring arrives, the lethargy lifts.
"It took me a while to realize what it was," said Balido, owner of a travel social network site, Tripatini. "I was cranky, short-tempered, depressed, feeling hopeless and having difficulty concentrating."
Balido, 51, was diagnosed a decade ago with seasonal affective disorder or SAD. The condition affects 62 million Americans, according to Michael Terman, director of the Center for Light Treatment and Biological Rhythms at Columbia University and a leader in the field.
About 5 percent of the population experiences the most severe symptoms of SAD -- depression and hopelessness -- while another 15 percent have the so-called "winter blues" or "winter doldrums."
The vast majority never fall into full depression, according to Terman, but "plod through winters with slowness and gloominess that takes effort to hide from others."
Two decades ago, SAD was identified as a legitimate disorder by the National Institute of Mental Health. Since then, the treatment of choice has been light therapy.
Balido, who lives in Miami, sought help from Terman and now undergoes light therapy. He sits in front of a daylight simulator for a half an hour each morning before 10 a.m.
"Within two or three days, the difference was mind-blowing," he said.
The standard treatment for SAD is 30 minutes of 10,000-lux, diffused, white fluorescent light, used early in the morning. About half the patients are helped quickly -- and when treatment is tailored to a person's individual wake-sleep cycle, remission can climb to 80 percent, according to Terman, author of the book on the subject, "Chronotherapy."
This year, a utility company in the northern Swedish town of Umea installed ultraviolet lights at 30 bus stops to combat the effects of winter darkness.
The dangerous effects of UV light have been filtered out.
"We wanted to celebrate the fact that all our electricity comes from green sources and we wanted to do this in a way that contributed to the citizens in one way or another," said Umea Energi marketing chief Anna Norrgard in an email to ABCNews.com.
"As it is very dark where we live this time of year, a lot of us are longing for the daylight," she said. "A lot of us are also a bit more tired this time of year and I would also say we sleep a little bit more. ...We wanted to give the citizens of Umea a little energy boost, to be more alert."
The town is located about 400 miles north of Stockholm. In December, the sun rises at about 10 a.m. and sets around 2:30 p.m. Some towns north of the Arctic Circle have no daylight for several weeks in the winter.
UV light does not provide relief for the winter blues, "per se," according to Terman, and might only provide a "transitory boost."
UV light acts on the skin to promote vitamin D production, but he noted that a body covered in winter coat, hat and gloves, would get little exposure to the beneficial rays.
Additionally, vitamin D production is not "key" to the antidepressant effect of white light therapy for SAD, according to Terman. Bona fide light therapy works through the eyes, not the skin.
But, he noted, geography has a strong influence on the prevalence of SAD symptoms.
"The common wisdom is that it's worse the farther north you live, because winter days are so much shorter," he said. "Not so simple."
Columbia research shows that in North America, the incidence of SAD rises from the southern to the middle states, but levels off and stays bad from about 38 degrees North latitude (near such cities as San Francisco, St. Louis and Washington, D.C.) up through the northernmost states and Canada, according to Terman.
But the problem becomes "more severe" at the western edges of the northern states and provinces.
"This important finding reveals the underlying trigger for relapses into winter depression, since the sun rises an hour more later at the western edge of a zone," said Terman, whose book, "Chronotherapy," looks at the phenomenon.
Esther Kane, a clinical counselor from Vancouver, Canada, said her practice is filled with patients as the long days descend on British Columbia.
"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.'"