Vitamin D deficiency is not just harmful to physical health -- it also might impact mental health, according to a team of researchers that has found a link between seasonal affective disorder, or SAD, and a lack of sunlight.

"Rather than being one of many factors, vitamin D could have a regulative role in the development of SAD," said Alan Stewart of the University of Georgia College of Education.

"Seasonal affective disorder is believed to affect up to 10 percent of the population, depending upon geographical location, and is a type of depression related to changes in season," said Stewart. "People with SAD have the same symptoms every year, starting in fall and continuing through the winter months.”

Stewart said, based on the team's investigations, vitamin D was likely to be a contributing factor in seasonal depression. "We believe there are several reasons for this, including that vitamin D levels fluctuate in the body seasonally, in direct relation to seasonally available sunlight," he said. "For example, studies show there is a lag of about eight weeks between the peak in intensity of ultraviolet radiation and the onset of SAD, and this correlates with the time it takes for UV radiation to be processed by the body into vitamin D.

Vitamin D is also involved in the synthesis of serotonin and dopamine within the brain, both chemicals linked to depression, according to the researchers. "Evidence exists that low levels of dopamine and serotonin are linked to depression, therefore it is logical that there may be a relationship between low levels of vitamin D and depressive symptoms. Studies have also found depressed patients commonly had lower levels of vitamin D."

"What we know now is that there are strong indications that maintaining adequate levels of vitamin D are also important for good mental health. A few minutes of sunlight exposure each day should be enough for most people to maintain an adequate vitamin D status."