system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 what "eye boogers" are, and what purpose do they serve?

My mom used to tell me I still had sleep in my eyes as I was eating cereal before school. When I asked what it was, she said it was dried tears from my yawns.

No idea if that's the real answer, but lately I've had trouble opening my eyes at all in the morning because I've got so much of it. 

Where does it come from and is there some biological or evolutionary purpose behind it?
gpt
In a nutshell, "Eye Boogers" ("rheum") is mucus mixed with blood cells, dust, skin cells etc. This mucus is called "Gound". This gound helps seal your eyes in an air tight fashion when they are closed, prevents tears from spilling onto your cheeks, and helps keep tears that coat your eyes from evaporating. 

Gound naturally disappears when blinking or on touch, but since you're not blinking at sleep whatsoever, this gound fills up and causes "eye boogers".