The Atlantic has a brief article about Seasonal Affective Disorder and how little is present in the nation of Iceland. The article attributes it to the quantity and quality of Omega 3 fatty acids in their diets. I found this quote interesting:
"It's not just the absolute amount of omega-3s that you take in, but also the amount relative to your levels of omega-6 fatty acids. The risk for depression may increase with a high ratio of omega-6 fatty acids -- found in corn, safflower, and soybean oil, and common in processed foods -- to omega-3s. "
Of course, those of us in the Paleo/Primal movement already knew that!