http://www.thisoldhouse.com/toh/article/0,,403815,00.html

Let’s start with the shape of a human head itself. Our heads are tapered downward from our cheekbones. Fixtures that just deliver light from the top of a mirror; only illuminate the top of your head. They produce a distinct shadow below your cheekbones and nose. This creates a less-than ideal reflection of the lower cheek, jaw line and mouth. The image below shows optimal light fixture selection and placement. Compare it to your makeup area. Is there a difference? Talk about “seeing the light.” Enjoy Your Project ~ ClearLED.biz

If there is room, then place the light(s) above the mirror. It also depends on the style of the light as well.