Why dogs sleep with their eyes open? If you have noticed your dog sleeping with his eyes open, consider that things may be deceiving. In reality, what you are likely seeing is your dog's third eyelid.

Why do dogs sleep with their eyes open? You may have stumbled in the past on some funny cartoon scenes where a guard dog is snoring with his eyes open, almost to attest that he's on guard duty 24/7 even when he looks asleep, but do dogs really sleep with their eyes open?

My dog sleeps with his eyes partially open from time-to-time and there are several dog owners who are wondering how and why dogs do it, so yes, dogs definitively sleep with their "eyes open" or semi-open but things are a tad bit different than what we may think. To better understand how and why dogs do this though, it's important to take a look at their anatomy.

Not What it Seems

We may assume dogs are sleeping with their eyes open, but in reality, they're not. Exposing their eyes to the elements when they are sound asleep would cause those eyes to dry up and become prone to damage. The only time when dogs are "sleeping" with their eyes truly open is when they are under anesthesia.

When I worked for a veterinary hospital, I saw my fair share of dogs who had their eyes wide open during surgery. As dogs can't blink as they're sound asleep on the surgery table, to prevent dryness to the dog's cornea (which can progress to a condition known as exposure keratitis), the vet would put a few drops of a special ointment.

The Third Eyelid

Dog with third eyelid showing

When you see your dog sleeping with his "eyes open" what you are truly seeing is his third eyelid.

Indeed, take a close look, most likely you won't see much of your dog's eye color but a light pink/red tissue covering the eye.

Also known as the nictitating membrane, palpebra tertia or haw, the third eyelid is a membrane that is drawn across the eye to protect the eyeball and keep it moist while the dog is sleeping.

According to the American College of Veterinary Ophthalmologists, it's estimated that the dog's third eyelid is responsible for the production of 40 to 50 percent of the dog's tears. This is thanks to a gland located by the third eyelid, known as the nictitans gland.

Additionally to keeping the eye lubricated when dogs sleep, the third eyelid prevents debris from attaching to the dog's eyes. The third eyelid indeed functions as an effective windshield wiper blade, sweeping foreign material off the eye, something that is very precious to animals who lack the manual dexterity to rubs their eyes to remove any foreign items, explains Dr. Eric Barchas.

Passive Range of Motion

Wondering how a dog's third eyelids works? There are no muscles attached to the dog's third eyelid.

Unlike cats, in dogs, movements of the third eyelid membrane are entirely passive.

Basically, the retraction of the dog's eyeball into the orbit elicits the third eyelid to passively slide across the ocular surface, explains, Christine C. Lim, a veterinary opthamologist in the book "Small Animal Ophthalmic Atlas and Guide."

Regardless if the dog is sleeping or not, the third eyelid may be more visible in certain breeds than others. It's mostly a matter of the relationship between the globe and the orbit.

In dogs with small eyes such as seen in dolichocephalic breeds (with long heads) like the borzoi and greyhound, the third eyelid will appear more prominent than in a brachycephalic breeds (with short heads) like say the bulldog or pug, points out Sheila Crispin, Past-President and Fellow of the Royal College of Veterinary Surgeons in the book: Notes on Veterinary Ophthalmology.

Eyes Wide Awake

When the dog opens his eyes after sleeping, the third eyelid should retract and assume its normal position, tucked out of sight at the inner corner of the eye.

With the dog awake now, he can resume blinking to keep debris off and keep the eye moist. When a dog though has a third eyelid showing even when awake, this can signal a problem.

The third eyelid may show in the case of an eye injury, pain or an illness or it could be a sign of a damaged nerve, explains veterinarian Betsy Brevitz in the book: "Hound Health Handbook: The Definitive Guide to Keeping Your Dog Happy." So if your dog's third eyelid is showing, it's best to see your vet for an evaluation.

Did you know? In humans, the third eyelid has shrunk to a rudimentary bump in the inner corner of the eye, explains veterinarian Paul Miller of the University of Wisconsin-Madison.