Living in Vermont, science and nature writer Rowan Jacobsen noticed the effects the seasons had on him. Summers were gorgeous, but the grey winters were brutal and left him lagging. “I felt as if my cells didn’t work,” he writes in his new book, “In Defense of Sunlight, The Surprising Science of Sun Exposure” (Scribner). But, during sunnier times, when he got some early morning rays, he writes “It was as if some nutrient was pouring from the sky.” While the sun and its UV rays have been