Most patients assume their dark circles are the result of dehydration, allergies, or a lack of sleep. The truth is that aging is the most common cause of dark circles. Since the skin under the eye area is naturally thinner than other areas of the face, aging causes the skin to become thinner due to fat and collagen loss. When this occurs, the skin starts to sag and the blood vessels under the tissue become more prominent and lead to a dark and sunken appearance. Men and women who were born with thinner skin are more prone to dark circles than those with naturally thicker skin.