Why Do Dogs Sleep with Eyes Open?

Have you ever wondered about the third eyelid? It's a fascinating structure that both humans and dogs once possessed. This eyelid, a part of the eye, is primarily responsible for keeping the eyes moist and wiping away any debris or dust that may gather around the eye.

Now, let's talk about the canine's most powerful sense—smell. While this sense is incredibly keen, it lacks precision without the aid of eyes. This is why the eyes are a crucial evolutionary feature for dogs.

Surprisingly, humans and dogs once had a third eyelid, which has since退化 into a small red bump in the corner of our eyes. However, in dogs, the third eyelid has retained a more robust function. Unlike in humans, the third eyelid in dogs is not controlled by the eye muscles and has even evolved its own set of muscles.

This means that the third eyelid in dogs operates independently, knowing exactly when to activate and when to rest. More importantly, it plays a vital role in maintaining the moisture in a dog's eyes.

But what does this have to do with dogs sleeping with their eyes open? When a dog sleeps, it closes its eyes, and the third eyelid wraps around the eye, protecting it from foreign particles. This is why you might sometimes see what appears to be the white of the dog's eye, but is actually the third eyelid at work.

As for why you can see the third eyelid in dogs, it all boils down to dreams. You've probably seen a dog dreaming, with its legs twitching and barking. During this time, the dog's eyes may slightly open. New dog owners might be startled by this sight.

Rest assured, this is a normal and protective mechanism at work. The brain allows a small gap in the eyes to remain open during deep sleep to prevent an attack from a predator. This is a common occurrence and nothing to worry about.

Some dog breeds, such as Pugs, French Bulldogs, Chihuahuas, and Yorkies, with their protruding eyes, are more prone to this. Simply put, this is a result of selective breeding for human aesthetic preferences, leading to eyes that cannot close properly or easily, causing the dog to sleep with its eyes open.