"Wipe the sleep out of your eyes". exactly, what does this mean?

When i was a boy getting ready for school each morning, my mother would tell me, “wipe the sleep out of your eyes”. i did not think about this then, but now, i am wondering exactly why she made this comment to me. was she telling me that other people will read my dreams, if i did not wipe the sleep out of my eyes? i know that “sleep” refers to that little white unknown buildup located on each side of our eyes. Question: so, how did this phrase come to be and is this just another “old wives tale”?