Rheum is a meical term for the natural watery discharge from the eyes, commonly known as "sleep", "eye cheese" or "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.