MISTER SANDMAN BRING ME A DREAM
The magical character who sprinkles fine grains of sand into our eyes to induce sleep first appeared in a story by Danish writer Hans Christian Andersen. The Sandman occurs repeatedly in folklore across the world, interpreted differently from stories to word of mouth, literature to film, and even to popular songs. While Andersen's Sandman was kind and told beautiful stories to children as they slept, some other versions of him are more sinister. In 1993 Paul Berry's short film "The Sandman" was nominated for an Oscar for Best Animated Short Film. In USA, the Sandman was used in a TV advert to "hypnotise" people into buying mattresses. Legend has it that the sand prevents the eyes from opening until morning so that children will have a good night's sleep. Upon waking, many people need to wipe away the grit that is often left in the corners of the eyes, "proof" of the Sandman's night-time visit. 
What is the substance that remains in the corners of the eyes and why is it there? It is called rheum, a thin mucous naturally discharged from the eyes, nose or mouth during sleep, and gathering as a crust in the corners of the eyes and mouth, and along the eyelids. Just like the mouth and inner nose, the outer layer of the eyes functions best when wet. The eyes are kept moist by a thin layer of tear film made up of water, mucous and oil produced by the meibomian glands in the eyelids. The oily meibum has numerous functions: as well as preventing the tear film from evaporating and the tears from streaming downwards, the moistness helps to focus incoming light by keeping the cornea smooth.
When we are awake, blinking of the eyelids causes rheum to be flushed away with tears via the nasolacrimal duct. When we close our eyes for a long night's sleep, the tears continue to wash over the eyes as they move around during different sleep phases. The tears pick up dust, dead cells, mucous and other small foreign objects in the eyes, and the closed eyelids guide this excess matter to the corners of the eyes and along the eyelashes where it accumulates.
Most people wake up with sleep crust, although it occurs in different amounts in different people. The amount may vary depending on the season or the general condition of the eyes and how dry they are. Sometimes eye discharge may be a sign of a problem with the eyes or eyelids, such as allergic conjunctivitis, infection or an inflammation of the eyelids. Be aware of the signs that may indicate a problem. These include changes in the quantity, colour or consistency of the discharge, increased light sensitivity, blurry vision, pain or redness of the eye. 
Practicing good eye hygiene is important in keeping the eyes healthy and avoiding irritation and infection. The best way to wipe sleep out of the eyes is with a cloth or cotton wool and warm water. Always remove makeup before going to sleep. It is tempting, but try to avoid rubbing the eyes, particularly on waking in the morning. If abnormal eye discharge is a symptom of conjunctivitis, which is very contagious, prevent it spreading by washing the hands frequently, not touching the eyes and not sharing towels with other family members.