Why do snakes sleep with their eyes open?

Why do snakes sleep with their eyes open? 

Answer: Snakes sleep with their eyes open because they dont have any eyelids. Their eyes are covered by a clear scale called a spectacle. . . You can see this when your snake sheds its skin. - Some scientists believe that snakes went through a period in their evolution when they were underground and lost their eyes completely.- That may be why snake eyes have a very different structure than any other kind of animal. they dont have eyelids.. but they still sleep..you can tell by looking at the eye..if its like a very thin line in the middle..they're asleep.. make sure u wake them up 1st by tapping the cage before picking it up..they get aggressive when u shock them .