Fact of the day… ”sleep” in the eyes. ”Sleep” is a combination of many different things: Oil, mucus, skin cells and other particles that gather and mix together when in slumber. The substance formed can be sticky, wet, crusty or dry dependent on how much of the liquid within the substance has evaporated. ”Sleep” is also commonly known as rheum, the discharge does actually have a key role in protecting your eyes, removing particles that may have entered the eye and could potentially cause damage. Our eyes produce mucus during the day however a continuous film of tears covering the eyes clears this when we blink, this flushes out the Rheum before it has the chance to harden in the eyes. The crusty substance builds up in the corner of our peepers during the night due to the fact that when we sleep our eyes are constantly shut, it also often forms along the lash line.