Also often called eye crusties, or sleepies, the official name is rheum.  Rheum is a natural discharge of mucus, tears, dust, and even skin cells from the eyelids.  During the day, blinking washes away the rheum, but at night it builds up because we don’t blink.  

Getting into more detail, there are several different types of eye boogers.  Each tells a story.  Not to be gross, but they are described as: round clumps, yellow crusty mucus, white stringy substance, or a thick greenish-grayish paste. 

Causes for excessive eye boogers are typically: clogged eyelid glands, infection of the tear drainage system, bacterial skin infections, viruses, pink eye, and excessive tearing from allergies, overactive tear ducts, etc.  Each type of eye booger typically corresponds with the cause of it.  

For example, thick, pasty greenish-grayish mucus may stem from a bacterial eye infection.  It may be difficult to open your eyes when waking up because pus has been produced overnight.   

Kinda makes you hungry, doesn’t it?