A cloudy day may disappoint some people, but for others, it causes a true biological change in their mood, behavior and cognitive functions.

Classified under the umbrella-term seasonal affective disorder, the condition comprises a group of symptoms similar to that of depression. seasonal affective disorder can be medically diagnosed and treated, but research is ongoing to determine what causes it .

“We don’t know for sure what causes SAD, but we have some hypotheses,” said Kelly Rohan, a psychiatry professor at the University of Vermont. “There is mixed evidence and no one [hypothesis] is supported.”

Rohan’s research focuses on treatment and the long-term effects of different kinds of treatment for seasonal affective disorder.  The results of one of her studies were published in the September 2009 issue of the Behavior Therapy journal.

Rohan said one hypothesis about the cause of the disorder involves the circadian rhythm. The circadian rhythm is a roughly 24-hour cycle closely linked to the dark and light cycle of the day.

It may be that the circadian rhythm of a person with seasonal affective disorder is disturbed, which can cause a change in hormones and therefore a change in mood, according to Rohan.

She said that one of the hormones involved in the circadian rhythm is melatonin, which is produced “during the night hours, from dusk to dawn.”

One function of melatonin is to regulate a person’s mood.

Rohan said past studies have shown the additional hours of darkness in winter cause an increase in melatonin production.

“There is an average difference of about 30 minutes in winter, which is enough for some people to make their hormones unbalanced,” Rohan said.

While many people link the effect of seasonal affective disorder with the time of year, one researcher said season is not a good classification for the condition’s causation.

Shia Kent, a biostatistics graduate student at the University of Alabama at Birmingham, studied the associations among stroke incidence, cognitive function and solar radiation energy—the amount of energy received from the sun at any given place, at a certain time.

Kent found a link between the amount of solar radiation energy and cognitive function. He said this makes him believe the effects of seasonal affective disorder are caused by solar energy, and while the sun is not as strong in the winter, a person can still have the disorder during both winter and summer, depending on the solar energy.

“Season maybe has more to do with the holidays and stress,” Kent said. “When I added season to my model [of correlations] there was not much change.”

Kent said that he believes the cognitive changes are mostly influenced by solar energy and radiation.

Another hypothesis, Rohan said, is that seasonal affective disorder is caused by a lack of light hitting the eyes of a person.

Ignacio Provencio, a biology professor at the University of Virginia, studied this hypothesis because one of the most widely-used treatments for seasonal affective disorder is light therapy.  During light therapy, a person with Seasonal Affective Disorder stands in front of a special light box for a certain amount of time each day.

“We thought perhaps the reason light therapy works is because people with SAD have some kind of sensitivity to light, which is overridden by light therapy,” Provencio said.

Provencio’s lab studies the eye, he said. They look at the rods and cones that most people know as part of the vision system.

“Rods and cones are photoreceptors, and there is another cell in the eye called ipRGC, which appears to be directly light sensitive,” Provencio said. “We looked at 130 patients with SAD and 90 people without SAD. We looked for mutations in the gene for the ipRGC cell.”

Provencio said the hypothesis was that a mutation of genes in the eye would cause a person to be more sensitive to light. The light sensitivity, he said, could cause someone to need more light to hit the eye in order to maintain a good mood, which means they could potentially experience Seasonal Affective Disorder.

The research found seven people with two mutated copies of this gene, and they could potentially experience seasonal affective disorder.

The research found seven people with two mutated copies of this gene, and they were all in the group of people who experience seasonal affective disorder, Provencio said, and others only had one mutated copy.

“This means that having a mutated copy of this gene is a recessive disorder,” Provencio said. “People with two bad copies have a six-fold greater chance of experiencing SAD, which is pretty dramatic.”

Provencio said the recessive disorder causes the mutated genes, which causes the light sensitivity and then leads to SAD.

Symptoms linked to seasonal affective disorder include lack of energy, hopelessness, oversleeping, anxiety, social withdrawal and weight gain.

Rohan said that no matter which hypothesis is correct—circadian rhythm disturbance or sensitivity to light—the effectiveness of light therapy, shown in her studies, supports both hypotheses.

“Light therapy can correct both circadian problems and photon problems,” Rohan said, but it doesn’t prove that either one is more correct.

“Light therapy makes SAD patients better,” Rohan said.

Rohan also said that seasonal affective disorder lies on a continuum of seasonality and most people probably experience some side effects.

“Studies have compared regions by latitude and people who have lived in the more-northern regions longer are more affected by SAD than the newcomers,” Rohan said.

Kent added the example that “SAD  is much more prevalent in Seattle versus Arizona.”

Location may be another factor in the causation of SAD, but Kent said seasonal affective disorder is a collection of symptoms.   And these symptoms can cause depression, but “it cannot be assumed that people become depressed for any particular reason.”

Norman Rosenthal performed the first clinical studies of seasonal affective disorder, Provencio said. Since his studies began in the 1980s, research continues to solve the mystery of what causes the condition.

tgleason@chroniclemail.com