PORTLAND – December is a time of cheer for many people, but the long nights and cold weather mark the beginning of a season of depression for many in Maine and across the nation.

Tuesday was the darkest day of the year – only about 9 hours of daylight – and the start of a long winter in Maine.

For the estimated 6 percent of Americans who suffer from seasonal affective disorder, winter means a depression that makes it difficult to function. Another 10 percent to 20 percent have a milder version.

It’s not uncommon for folks to have an occasional bout with the blues, but SAD is a sustained syndrome, said Dr. Mark Fulton, a psychiatrist at Spring Harbor Hospital in South Portland.

“Some will, without prompting, tell you, ‘Doc, every year, this time of the year, I feel depressed,”‘ he said.

The cause of SAD isn’t known for certain, but many researchers believe it is a biological one related to lack of sunlight.

Circadian rhythms and the hormone melatonin may be involved, but that connection doesn’t always hold up, says Kelly J. Rohan, professor of clinical psychology at the Uniformed Services University of the Health Sciences, the military medical school in Bethesda, Md.

“It’s not as neat as everybody may want it to be,” said Rohan, who earned her doctorate at the University of Maine. “Other studies don’t find them.”

The theory that the lack of daylight hours is a cause is supported by findings that show the disorder is more prevalent in northern areas.

Ten percent to 15 percent of New Englanders have SAD, while there are few cases in Florida, says Kathryn Low, a professor of psychology at Bates College. “As you get closer to the equator, there’s very little of it,” she said.

Treatment using special lights is the most common, with the patient sitting in front of the lamp at prescribed times for a prescribed number of minutes. The lights are typically 20 times brighter than the light in an office setting, Rohan says.

Patients can start seeing results within three or four days, says Dr. Michael Garnett of Community Counseling Center. For someone who doesn’t have full-blown SAD, he says, that treatment is not going to help much.

Those suffering from SAD can take heart in the fact that the days will seem longer after Tuesday, which had only 8 hours and 54 minutes of daylight.

“That’s the point where we start heading toward spring,” said Eric Sinsabaugh, a meteorologist with the National Weather Service in Gray, “and the day starts getting longer.”