Why Is There Sand in Our Eyes When We Wake Up?
The “sand” that we sometimes find in the corners of our eyes when we wake up in the morning comes from a clear, thick fluid called mucus. The mucus is produced by the lining of the eyelid. This slippery fluid mixes with our tears and provides lubrication for the eyelid.
It also helps to keep dust and other foreign matter from entering the eyes and irritating them. While we are sleeping, our tears collect in the corners of our eyes. The mucus they contain begins to dry out. What’s left is that crusty stuff we rub from our eyes the next morning.
The sand is often most pronounced in children, so it’s no surprise the term “eye boogers” emerged. Fresh eye goop is yellowish and squishy, but once dried; it hardens and can resist removal by even the most determined finger.
Before it balls into eye grit, the fluid enables and enhances vision. German physician Heinrich Meibom discovered the meibomian glands in the eyelids in the seventeenth century. The glands create meibum, oil that coats the eye.
Just like the mouth and inner nose, the outer layer of the eye works best when wet. A thin layer of tear film, a fluid made of oil, water and mucus, keeps the eye moist. The oily meibum prevents tear film from evaporating between blinks. It also dams tears, preventing a lachrymal waterfall from streaming downward.
Aside from retaining moisture, the meibum shields the eye and helps to focus incoming light onto the lens. If the outside of the eye were dry, it would not be smooth; an eye as gritty as sandpaper would scatter light and make it unfocused on the lens.
During sleep, the eyes close, the eyelids kiss and the meibum gums together; it collects in a pouch usually reserved for tears and flows toward the nose, snowballing into a pill of oily dead skin and dust. When morning comes, the sandman is nowhere to be seen, but the meibomian glands continue to produce their oily elixir, enabling sight for another day.