Question: why do we get sleep in our eye?

Claire Donald answered on 21 Jun 2018:

Our eyes are covered in 3 thin layers that protect our eyes and allow them to work properly. There is a layer of mucus which attracts water, a layer of tears which keep the eye hydrated and keeps out infections, and a final layer made up of lipids like fatty acids and cholesterol. This top layer of lipids is what causes ‘sleep’. At body temperature (37C), it is clear and fluid, but at night when you sleep your body cools slightly which causes the oily fluid to solidify and become a waxy solid. Et voila- eye gunk!