Wearing sunglasses increases the risk of getting sunburned.

The reason for this is that your pituitary gland only knows how much melanin (pigment that protects your skin from ultraviolet light) to produce from the signals that it receives from the eyes.

If your eyes indicate that it is sunny outside, the gland produces a lot of pigment. However, sunglasses trick the gland into thinking its less sunny, meaning less melanin and a higher risk of burning!

(source)