When you put on your dark shades, your brain is under the impression that the light in that area is lesser than what it really is. Now because of this false impression of the brain, the pituitary gland isn’t triggered. When triggered by the right nerve, the pituitary gland releases a Melanocyte Stimulating Hormone (MSH). This hormone is responsible for creating melanocyte cells, the ones responsible for Melanin.
Melanin gives the skin its color. More the melanin, darker the skin. It also serves as a protection layer from the deadly UV rays of the sun. Without it, skin diseases and cancer would be common.
When you are falsifying the brain by wearing sunglasses, melanin isn’t produced and instead of getting tanned, your skin could get sunburned. Without the shades, the right light will release the right amount of melanin into your skin and you will get tanned because of the excess melanin. At least that’s better than getting burnt.