Why do we get sleep in our eyes?
Jason G Goldman
Our eyes fill up with gunk as we sleep, but what is it? As Jason G Goldman discovers, the stuff has a more important job than it seems.
The first thing I do when I wake up each morning is look at the long list of notifications that have silently accumulated on my phone as I slept. The second thing I do is wipe away the gunk that silently accumulated in the corner of my eyes over the course of the night. No matter what you call it – sleep, sand, eye boogers – you know the stuff I'm talking about. I've always wondered what it was made of and why it forms, so I went and found out.
It all begins with tears – or more precisely the tear film that coats our eyes. Mammalian eyes of the terrestrial variety, whether they're found on the faces of humans, dogs, hedgehogs, or elephants, are coated in a three-layered tear film that allows the eyes to function properly. (Tears work somewhat differently in marine mammals like dolphins and sea lions.)
Closest to the eye is the glycocalyx layer – a layer made mostly of mucus. It coats the cornea and attracts water, which allows for the even distribution of the second layer: the water-based tear solution. It might be just four micrometres thick – about as thick as a single strand of spider silk – but this layer is very important. It keeps our eyes lubricated and washes away potential infections. Finally, there is an outer layer composed of an oily substance called meibum, which is composed of lipids like fatty acids and cholesterol.
Meibum has evolved to be exquisitely tuned to the mammalian body. At normal human body temperature, it is a clear oily fluid. At just one degree cooler, though, it becomes a white, waxy solid – the familiar eye gunk.
Large flakes of this solid can form during sleep for a couple of reasons. First, the body cools down a bit at night in general, so some of the meibum becomes cool enough that it moves below the melting point and turns solid. Second, according to Australian ophthalmologist Robert G. Linton and colleagues, "sleep relaxes the [muscular] action on the [meibomian] gland ducts…[which] is sufficient to cause far in excess of the normal to exude onto the lids and eyelash roots during sleep". In other words, our eyes are coated with more meibum than usual at night – and so when that meibum cools we can end up with appreciable amounts of eye gunk.
It’s not too much of an inconvenience to wipe away the sleep when we rise, of course, but why do we make meibum in the first place? Well for one thing, it prevents our tears from constantly spilling out of our eyes and dribbling down our cheeks. Everyday tasks can be made more difficult with your eyes constantly streaming, as some people with hay fever will appreciate. By keeping our tears in our eyes, meibum does something else: it helps keep our eyes moist. In fact, one study found that, when deprived of meibum, rabbits' eyes lost water through evaporation at 17 times the normal rate.
Meibum is not the only factor that helps prevent our eyes from drying out. Blinking is important too. That’s because the act of blinking effectively milks your meibomian glands, causing a little bit of extra meibum to be expressed onto your eye in addition to the constant, steady secretion of the stuff. Blinking also helps the oily meibum and water tears mix together to form an emulsion called tear film. If you go for too long without blinking, the emulsion falls apart – oil and water don’t like to mix – and your cornea could become exposed to air. At best that’s uncomfortable – at worst, the chronic deterioration of the tear film could lead to a condition known as "dry eye" or, more technically, keratoconjunctivitis sicca.
Japanese ophthalmologist Eiki Goto refers to dry eye as a "major tear deficiency disorder" that affects millions of people around the world. In addition to dryness, the disorder leads to eye fatigue, red eye, irritation, and the sensation that the eyes are heavier than normal. In its most severe – but rare – forms, dry eye can even impair vision. Despite the irritation and frustration associated with the condition and its impacts on quality of life, it has not been historically considered a serious visual disorder.
Goto, however, disagrees. Using a sophisticated form of visual acuity testing, he discovered that the eye loses its smooth surface when its fluid coating dries out. Optical aberrations then become more common, because light is more likely to scatter of a rough surface, making it harder to form a sharp image on the retina.
This could explain another of Goto’s findings. He noticed that patients with the dry eye disorder tend to blink almost twice as often as those with normal ocular lubrication. It’s possible they do so because they are trying unconsciously to maintain their visual acuity.
You might think that this work suggests an easy solution to dry eye disorder: just blink as often as possible. Unfortunately, that’s easier said than done in our modern world. Many daily tasks – reading, driving, typing on a smart phone, and working on a computer screen – encourage us to sustain an unblinking gaze. Consequently, all of us reflexively reduce the frequency of our blinks during those activities.
For example, as we drive faster – especially at speeds exceeding 100km/h (62mph) – we blink less frequently. For dry eye patients, that means their functional visual acuity while driving at high speeds may actually drop below the required minimum to attain a driver's license. In another of Goto’s studies, the average functional visual acuity in dry eye patients was 0.3. That's less than the acuity of 0.7 that's required to drive in Japan and of 0.5 that's required to get a licence in the US. "This suggests that actual visual acuity while driving may be not adequate in certain patient groups," writes Goto.
So the next time you wake up and work to dislodge all that gunk from your eyes, perhaps you'll take a minute to consider just how important the stuff really is.