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"We hypothesize that rather than functioning primarily as a proximal or direct sub-mechanism in the etiology of SAD,vitamin D likely functions in a more foundational and regulative role in potentiating the sub-mechanisms associated with the depressive and seasonality factors," say the researchers.

Firstly, the researchers note that vitamin D levels in the body fluctuate with the changing seasons in response to available sunlight.

"For example," says Stewart, "studies show there is a lag of about 8 weeks between the peak in intensity of ultraviolet (UV) 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."

Co-author Michael Kimlin, of the Queensland University of Technology in Australia, says that vitamin D also plays a part in the synthesis of both dopamine and serotonin, noting that past research has associated low levels of these neurotransmitters with depression.

"Therefore," he adds, "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."