Why sloths do not get crusty eyes from all the sleeping

Those who spent the Christmas break catching up on lost sleep will share the familiar feeling of waking up with crusty eyes. Sleep dust, or rheum as it is more formally known, is really dried mucus, dust, skin and blood cells that are caught in or discharged by the eyes, and which dry out overnight.

Surely, you would think, sleep dust cannot be unique to humans? And if any animal is going to get crusty eyes, it would be the notoriously lethargic sloths. After all, they spend even more time asleep than we do.

So, do sloths get crusty eyes from sleeping all the time? We put this question to our readers to see what they thought.

"I wouldn't think so," says Laurie Thornburgh. "Nature has a way of adapting so they probably have something that keeps the crust from building up."

There is a logic to this. Crusty eyes may seem trivial to us, but they might hinder a sloth that was swinging around in the treetops, albeit in slow motion. However, researchers have found that sloths have such poor eyesight that they hardly use it for climbing or foraging. Instead, they rely on their superior sense of smell.

They produce this weird white goop on top of their eyes and nose

Briana Kortbein suspects that sloths might get crusty eyes after all. "Yes - it just runs down their face to add to all the other nastiness they have all over them... Kind of like my parent's little dog."

We contacted researcher Becky Cliffe of the Sloth Sanctuary of Costa Rica. She says that sloths do get stuff in their eyes sometimes, but it is different to our rheum.

"I've seen lots of three-toed sloths in the wild secrete a red substance from their eyes," says Cliffe. "They eat a type of leaf called Cecropia, which is green but has a red pigmentation within it. It's also prominent in pygmy three-toed sloths, because they eat lots of red mangrove leaves."

This is where things get a little more complex, because there are two groups of sloths: three-toed sloths and two-toed sloths. They are separated by over 15 million years of evolutionary history. Three-toed sloths secrete red gunk from their eyes, but what about two-toed sloths?

"Here's a really weird one for you," says Cliffe excitedly. "This is completely unknown to science." When [two-toed sloths] get excited, either sexually or for another reason, "they produce this weird white goop on top of their eyes and nose. It looks terrifying, and when their stimulant passes they just reabsorb it."

Crusty eyes can occasionally be seen in sloths, if they've been poked in the eye by a leaf or twig

Cliffe has not yet studied the cellular makeup of this white goop, but she suspects that it may contain pheromones: stimulating hormones that waft through the air. Members of her team have wiped this mysterious goop on cotton buds and waved it under the noses of other male and female sloths. Sometimes the other sloths began secreting it as well, but not always.

This phenomenon has barely even been mentioned in the scientific literature, perhaps because sloths are surprisingly under-researched.

However, zookeeper Kelly-Anne Kelleher of the Zoological Society of London has noticed something similar in captive two-toed sloths. "When sloths are excited, I've seen small whitish beads of fluid on their noses," she says. "It is likely just sweat. This can dry and leave watermarks on their faces."

It is starting to sound like sloths do get crusty eyes, but not for the same reason we do.

"Their eyes look fairly clean when you look at them up close," says Katia Hougaard. "I've only seen crusty eyes on a sloth with health issues."

Similarly, Richard Spencer says "they would only get weepy eyes when they are sick just like dogs do."

People think they're sleeping all the time because they can't see them move

"Crusty eyes can occasionally be seen in sloths, if they've been poked in the eye by a leaf or twig while climbing through the treetops," Kelleher says. "You can also see it sometimes if the relative humidity drops below about 65-70%."

Whatever is happening, crusty eyes in sloths do not seem to be caused by sleeping a lot, as may be the case with humans. Cliffe says part of the explanation for this is that sloths are not as sleepy as everyone thinks.

"Sloths don't actually sleep that much," says Cliffe. "It's a complete myth. Two-toed sloths sleep for about 8-9 hours a day in the wild." For many of us that may still sound like a pretty fantastic amount of sleep, but they are not exactly sleeping all day.

"Most of the time they're just moving really slowly," says Cliffe. "Everything they do is at the same measured pace. People think they're sleeping all the time because they can't see them move, and that's the whole point: so they go unnoticed."

As for three-toed sloths, Cliffe has studied 30 individuals in the wild and each one had a different sleeping pattern.

A 2014 study by Niels Rattenborg and colleagues from the Max Planck Institute of Ornithology found something similar. The sloths he studied slept, woke up, and moved as and when they needed to. They did not really have a circadian rhythm distinguishing their behaviour in the day and night, except that mainland sloths showed a preference for sleeping at night but island dwellers had no preference.

I'd be a little concerned trying to clean up the crusty eyes with those claws

All this thought of sleep has Sergey Ayrapetyan wondering "how sloths sleep at the [height] of 30 metres and don't [fall] down?"

Lucky for sloths, their claws work in the opposite way to our hands. Their default position is a tight strong grip, and they must exert effort to open them. That means sloths do not let go of their branches when they fall asleep, like we would.

We use the word "claws" advisedly, because sloth claws are not – as in many other species – essentially long, hard fingernails, but are actually the sloth's protruding finger bones. These appendages give the sloths a very fine sense of touch, which in some ways compensates for their poor eyesight.

On the whole, it is probably for the best that sloths do not get rheum in their eyes from sleeping like we do, because their claws are extremely sharp. "I'd be a little concerned trying to clean up the crusty eyes with those claws," says Leslie Da Lie.

Join over six million BBC Earth fans by liking us on Facebook, or follow us on Twitter and Instagram.

If you liked this story, sign up for the weekly bbc.com features newsletter called "If You Only Read 6 Things This Week". A handpicked selection of stories from BBC Capital, Culture, Earth Future and Travel, delivered to your inbox every Friday.