Your morning eye boogers’ role in well-lubricated eyes
Nothing says “I’m waking up” like rubbing the sleep, eye boogers, etc. out of your eyes. The nearly experience is nearly universal among land mammals, so much so that most of us take it for granted at this point. If you stop and think about it though, it’s actually sort of puzzling that the crusty buildup in the corner of your eye is something that’s actually there to help keep your eye smooth and lubricated.
Before we get to the crusty eye boogers, we need to backup a bit. Our eyes are covered in three layers of liquid film, each with different roles that support each other in the task of keeping our eyeballs clean and lubricated. This is really important, because if eyeballs that are too dry, because of a physiological issue or just staring too long, actually function worse. The surface becomes uneven, which bends and distorts the light entering the eye. So much of what follows are ways to keep a nice, even shape to our eye, free of distortion or obstructions.
Layers of liquid eyeball protection
The closest layer to the eye is more or less mucus. Its job is to help retain moisture on the eyeball. That moisture is generally kept in the second layer, which is where your tears come in to play. That four micrometer layer of tears helps wash out debris and sources of infection as well as lubricate the eye. While the mucus layer helps keep the tears in place to a degree, everything is capped off with the meibum layer, which is a substance made of fatty acids and cholesterol. That layer helps keep the tears in place (instead of constantly dribbling down your face) as well as helping keep the distribution of moisture even across the eyeball.
So which one of these three ingredients is dry and crusty when we wake up? It turns out that the name “eye booger” is a bit of a red herring, as its not mucus you rub off your tear duct, it’s the outer meibum layer. Meibum is very sensitive to temperature shifts, and cooling down even 1 degree can push it from liquid to solid, where it becomes flaky and cloudy. At night, your temperature drops slightly, and the muscles helping distribute the protective films across your eye rest, allowing the solidified meibum to build up while you sleep.
Fortunately, it doesn’t take much warming up to get your eyeball up lubricated again in the morning.
My kindergartner asked: If the meibum helps keep the tear layer in place, why do tears roll down your face when you cry? Tears are actually a bit complicated in their own right, but they can outdo the meibum layer when production is kicked up because you’re eye is trying to flush debris or you’re experiencing intense emotion.
Source: Why do we get sleep in our eyes? by Jason G. Goldman, BBC Future