Rheum

Rheum is a thin mucus naturally discharged from the eyes, nose, or mouth, often during sleep. Rheum dries and gathers as a crust in the corners of the eyes or the mouth, on the eyelids, or under the nose. It is formed by a combination of mucus, nasal mucus, blood cells, skin cells, or dust.
Rheum from the eyes is particularly common. Dried rheum near the eyes is commonly called 'sleep', 'sleepy-seeds', 'sleepy buds', 'sleepy sand', 'eye boogers', 'eye goop', 'sleepies', or 'eye gunk'. When the individual is awake, blinking of the eyelid causes rheum to be washed away with tears via the nasolacrimal duct. The absence of this action during sleep, however, results in a small amount of dry rheum accumulating in corners of the eye.

Medical conditions

A number of conditions can increase the production of rheum in the eye. In the case of allergic conjunctivitis, the buildup of rheum can be considerable, preventing the patient from opening one or both of the eyes upon waking without prior cleansing of the eye area. The presence of pus in an instance of heavy rheum buildup can indicate dry eye or conjunctivitis, among other infections.