What is Sleep in Your Eyes?

Sleep, otherwise known as sleep dust, eye sand or eye crust, is a mixture of skin cells, mucus, oils and other eye debris. It collects in the corner of our eyes whilst we sleep and we often need to clean our eyes to get rid of it. Whilst sleep has many colloquial names, it is technically referred to as rheum.

The consistency and texture of sleep, whether it’s wet or dry for example, depends on how much of the liquid has been evaporated. It is exactly for this reason, it’s often known as eye crust as the evaporation leaves a hardened layer of gunk. Though it’s largely a natural and harmless phenomenon, sleep can be a symptom of infection, especially if you notice a change in the amount, consistency and colour.

Why do we get sleep in our eyes?

We collect sleep because when we’re asleep and not blinking, eye debris isn’t as effectively washed away. This means it can accumulate in the inner corners and lash lines of our eyes.

For a more comprehensive answer, we’ll need to dig into the structure of our eyes. Terrestrial mammals (mammals that live on land) such as arctic foxes and zebras, all have the same three-layered ‘tear film’ that sits on the top of their cornea in their eye to protect it from the outside. The tear film in total is only 3 micrometres (0.0003 cm) thick, yet it is crucial for healthy eye function. The first layer, the glycocalyx layer, is made primarily of mucus which allows for the equal distribution of the second layer, the tear solution, which keeps our eyes lubricated and washes away infections. The outer layer is made of meibum, an oily substance made from fats, which prevents our tears from simply evaporating away.

When we sleep, our body temperature cools and this can be enough to start to solidify some of the meibum. Sleep also relaxes our meibomian glands causing an increase in the production of meibum, the surplus of which can spill out into our eyes’ corners, lids and eyelashes. Resultingly, we get a build-up of sleep.

Should you worry about the build-up of sleep?

The presence of rheum in your eyes is normal, but that doesn’t mean you shouldn’t pay attention to it if it changes in quantity, colour and consistency. Changes to your sleep are sometimes indicators of an infection. Here are some warning signs you should look out for:

Increased quantity of sleep (notably when your eyes become bunged up)
Changes in sleep colour to a green-yellow or white
Changes in texture from a crust to moist or ‘wet’ sleep
Eyelids sticking together causing difficulty when opening your eyes
Blurred vision
Sensitivity to light
Puffiness, redness and swelling of your eye
Eye pain

It is important you consult your doctor as soon as you encounter these symptoms.

Young girl looking through a magnifying glass

Common eye infections that affect sleep

Conjunctivitis

Conjunctivitis, also known as pink eye, is an infection of the thin membrane that covers the white of your eye, forming a white, yellow or green puss that sticks to your eyelashes. Additionally, it can also cause your eyes to turn red, and itch. It may even cause your eyes to feel gritty. There are 3 types of conjunctivitis: viral, bacterial and allergic. They all have slight differences in eye discharge symptoms. Viral conjunctivitis is commonly associated with clear eye discharge or slightly white or yellow, bacterial tends to be more yellowy, green or grey and allergic conjunctivitis tends to be watery. Recovering from conjunctivitis usually takes a couple of weeks.

Blepharitis

Blepharitis is inflammation of the eyelids caused by a clogging of oil glands located at the base of the lashes. Blepharitis causes swollen, itchy eyelids and can cause crusty flakes to appear on your eyelashes. However, it is usually not serious and can be treated by cleaning your eyelids.

Dry eyes

Dry eyes are commonly caused by inadequate tear production or a problem with your meibomian glands causing your eyes to be insufficiently lubricated. This causes them to feel dry resulting in irritation and inflammation. Common symptoms include sore, itchy eyes that may also be more watery – despite the feeling of dryness.

How to clean sleep from your eyes

Here are some top tips on how to clean sleep from your eyes, recommended by the NHS:

Wash your hands thoroughly to avoid transmitting infection from your hands to your eyes
Soak a flannel in warm water, but not hot water as it can damage your eyes, then carefully press it onto the area surrounding your eyes
Delicately massage your eyelids with your fingers or a cotton bud
Soak cotton wool in warm water and gently wipe away excess gunk to clean your eyelids

Sleep in Folklore: The Sandman

The origins of referring to sleep as ‘sand’ come from the mythological character of Mr Sandman. We know him as a benevolent person who sprinkles sand in our eyes so that we sleep well at night and have lovely dreams. However, his origins are a great deal more sinister.

Mr Sandman comes from the tale of ‘Der Sandmann’, a short story written in 1816 by Gothic author Ernst Hoffman. The tale goes that he is a wicked man who throws sand in the eyes of children who won’t sleep. The sand causes the eyes to fall out which are then collected by The Sandman who puts them in his bag before feeding them to his children located at an iron nest on the moon! This legend was deemed necessary to encourage children to sleep so that they would be well rested to help with chores. That was the theory anyway but we think it must have given them nightmares!

Contrary to Hoffman’s sinister portrayal of The Sandman, Hans Christian Anderson, creator of some of our most beloved fairy tales including The Little Mermaid and The Princess and the Pea, introduces The Sandman as a more compassionate character. In his 1841 tale, Ole Lukøje, the Sandman is named Ole Lukøje, literally, Ole Close-Eye, who, instead of wanting to hurt the children, instead sprinkles dust so that children are asleep and quiet so that he can tell them stories.

You know that Hoffman’s Sandman is sadistic, when even Hans Christian Anderson, known for his odd, twisted and creepy stories, is honeyed in comparison. Times change, however, and thankfully the story has taken a much less macabre twist.