Eyelid
An eyelid was a flap of tissue that protected an individual's eyes from irritants and light.

Vulcans were known to have an inner eyelid which protected their eyes from the harsh sunlight on planet . While traversing Vulcan's Forge, T'Pol assured Jonathan Archer that hers would protect her from the sun's glare. ()

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 of 1,000,000 candles per square inch for several seconds, intended to free his nervous system from a flying parasite. ()

Xaheans had interior eyelids to the left and right of their eyes, invisible unless they blinked. (; )

In 2368, Geordi La Forge remarked to Hannah Bates that he felt as though his eyelids had lead weights attached. ()

In 2372, when Jake Sisko tried to awaken his grandfather from a nap, the older man facetiously asserted that he was not sleeping, just checking his eyelids for holes. ()

Displacement of the eyelids was a possible symptom of Tuvan Syndrome. In 2375, Doctor Julian Bashir told Luther Sloan that Koval had this symptom. ()

See also
 Nictitating membrane

External link 
 

de:Inneres Augenlid
it:Palpebra interna
Anatomy
Sleep