The idea that sunglasses might increase risk of skin cancer comes from a British physician who published a book on the topic in 2007. The gist of his notion is that reducing the amount of ultraviolet (UV) light reaching key areas of the eye could trick the brain into believing that fewer harmful rays have penetrated the body. In response to this mistaken perception, the body might then produce less melanocyte-stimulating hormone, the chemical signal that thickens and darkens the skin as a defense against UV damage.
At the time this warning was issued, some British dermatologists signed on to the idea that sunglasses can decrease the skin's natural tanning response, although they cautioned that further research would be needed to prove it. Some also suggested that clear, not dark, sunglasses would be preferable (colorless UV-protective coatings are available).
In the years since this theory was advanced, I haven't seen any research supporting it. Indeed, UV-blocking sunglasses with wraparound or large frames are still recommended by the Skin Cancer Foundation to protect the eyelids and the sensitive skin around the eyes, which are common sites for skin cancer and sun-induced aging.
At this point, I see no reason to give up wearing sunglasses, especially as they also reduce risk of cataract and macular degeneration, common causes of age-related loss of vision. Your best defenses against skin cancer are staying out of the sun when it's at a high angle in the sky, wearing hats and protective clothing, and using sunscreen with an SPF of 30 or higher (apply it at least 15 minutes before going out into the sun and reapply it every two hours). I prefer sunscreens with Z-COTE, a form of microfine zinc oxide.
However, don't avoid the sun entirely: you need some exposure to optimize levels of vitamin D.
Andrew Weil, M.D.