The reason that the undereye area appears dark is because of the thin skin in that area, so keeping the area moisturized and "plumped" helps. Sleep hydrates the skin, so dehydration and lack of sleep causes the area to appear darker. Iron deficiency can also contribute to darkness around the eye area. When applying anything around the eye area though, be careful to apply it along the lower part of the orbital bone, and not too close to the eye. The warmth from the skin causes it to "move", and get into the tear duct, especially while sleeping, which can result in puffiness.