Rheum is a medical term for the natural watery discharge from the eyes, commonly known as sleepydust, eye boogers, or some derivative thereof, which forms a crust on the eyelids during sleep (contrast to mucopurulent discharge). It is formed by a combination of mucus (consisting of mucin discharged from the cornea or conjunctiva), tears, leaked blood cells, dead skin cells from the eyelids, and dust.
Normally, blinking causes this substance to be washed away with tears. The absence of this function during sleep, however, results in a small amount of dry rheum forming in the corners of the eyes even among healthy individuals, especially children. Still, the formation of a large amount of crust or the presence of pus within it may indicate dry eye or other more serious eye infections including conjunctivitis and corneitis.
Adults and older children can easily remove the crust by washing the eye with water or simply brushing them away with clean fingers. In young children, however, the buildup of rheum can be so severe, that opening one's eye upon awakening can be difficult or impossible without washing the eye. Very young children or people under care may need to have this done by another individual.
Rheum can eventually lead to serious eye disease if it occurs in large amounts over time. Large amounts of mucus and crust in the eye may signal a serious infection.