A snake can’t close its eyes because it has no eyelids. Instead of a movable lid to protect its eyeball, a snake has a hard, transparent shell over its eyes, almost like a pane of glass.
This shell protects the eye, yet doesn’t block the snake’s vision. But a snake never closes its eyes. So scientists cannot definitely determine when it sleeps or hibernates.
A snake sheds its eye covering each time it sheds its skin!