Vulcans have a second set of inner eyelids, which they evolved to protect their eyes from the harsh sunlight on planet Vulcan. Many species throughout the galaxy, both sentient and not, have this characteristic (known in anatomy and biology as a nictitating membrane).
While on Vulcan investigating an attack on the United Earth Embassy in 2154, Captain Archer had to wear sun glasses while traversing the desert, while T'Pol did not need them, thanks to her inner eyelids. (ENT: "The Forge")
In 2267, on a mission to the Deneva colony, Spock's inner eyelids enabled him to completely recover his sight after being blinded by a bright light dose intended to free him from a parasite. (TOS: "Operation -- Annihilate!")