Cats squint their eyes for two very different reasons: as a deliberate social signal communicating trust and comfort, or as an involuntary response to eye pain, irritation, or bright light. The difference matters because one is a sign your cat feels safe around you, while the other can indicate a problem that needs attention.
The Slow Blink: A Sign of Trust
The most common reason a healthy cat squints is the “slow blink sequence,” a behavior that functions as positive emotional communication between cats and humans. It typically involves a series of half-blinks followed by a prolonged eye narrowing or full eye closure. You’ve probably seen it: your cat looks at you, slowly narrows their eyes, and holds them partly or fully shut for a moment before opening them again.
A 2020 study published in Scientific Reports confirmed what cat owners have long suspected. In the first experiment, cats produced more half-blinks and eye narrowing when their owners slow-blinked at them compared to when there was no interaction at all. In a second experiment using an unfamiliar person, cats were more likely to approach the stranger after receiving a slow blink than when the person maintained a neutral expression. The findings suggest slow blinking works as a genuine two-way signal, not just a coincidence.
Narrowing the eyes appears to be tied to positive emotions across many species. Dogs narrow their eyes during play, horses and cows do it while being stroked, and in humans, the genuine smile (called a Duchenne smile) involves the same kind of eye narrowing. For cats, slow blinking seems to serve a similar role: it signals relaxation and non-threat. You can try it yourself. Slow-blink at your cat from across the room, and there’s a good chance they’ll blink back.
Squinting From Bright Light
Cats are far more sensitive to light than humans. Thanks to a reflective layer behind the retina called the tapetum lucidum, a cat’s light sensitivity is roughly six times greater than yours. This is what gives them excellent night vision, but it also means sudden bright light can be uncomfortable. A cat stepping from a dim room into direct sunlight will often squint as a natural reflex to reduce the amount of light hitting the retina, working alongside pupil constriction to regulate brightness. This type of squinting is brief, situational, and nothing to worry about.
When Squinting Signals Pain
Persistent squinting in one or both eyes is one of the clearest signs of eye pain or irritation in cats. Veterinarians use a standardized tool called the Feline Grimace Scale to assess pain, and “orbital tightening” (squinting or tensing around the eye) is one of its five key indicators, alongside ear position, muzzle tension, whisker changes, and head position. If your cat is holding one eye partially closed for hours rather than minutes, that’s a red flag.
Several conditions cause this kind of involuntary squinting:
- Corneal ulcers are one of the most common culprits. A scratch from a fight, an ingrown eyelash, trapped debris under the eyelid, or a viral infection can all damage the clear outer surface of the eye. Signs include discharge, cloudiness, sensitivity to light, and pawing at the face. The most frequent cause, according to Cornell University’s College of Veterinary Medicine, is recurrent infection with feline herpesvirus.
- Conjunctivitis (inflammation of the tissue surrounding the eye) causes redness, swelling, and discharge that makes cats squint and rub at their eyes.
- Uveitis is inflammation inside the eyeball itself, and its signs include squinting, swollen third eyelids, and visibly enlarged eyes.
A healthy cat’s eyes should be bright and clear, with equally sized pupils and no squinting from either eye. Squinting that’s limited to one eye is especially telling, since it usually points to a localized problem on that side.
The Third Eyelid and Protective Reflexes
Cats have a built-in piece of eye protection that most people rarely notice: the third eyelid, or nictitating membrane. It’s a transparent eyelid that sweeps horizontally across the eye to moisten and shield it. You might catch a glimpse of it as a white or pinkish film in the inner corner of your cat’s eye when they’re very relaxed or just waking up.
When a cat squints from pain or irritation, the third eyelid often becomes more visible as part of the protective reflex. If you notice the third eyelid prominently covering part of the eye during normal waking hours, that’s another signal something is off, whether it’s eye pain, illness, or a condition called Haws syndrome where the third eyelids elevate on both sides.
Flat-Faced Breeds and Chronic Squinting
Persian cats and other flat-faced (brachycephalic) breeds are structurally prone to eye problems that can cause ongoing squinting. Their shortened skulls create shallow eye sockets, which makes the eyes more prominent and exposed. This leads to several overlapping issues.
Entropion, where the eyelid rolls inward so that fur constantly rubs against the cornea, is one of the most significant. The chronic irritation it causes can progress to corneal ulcers if left untreated. Brachycephalic cats also tend to have decreased corneal sensitivity compared to cats with normal skull shapes, which means they may not react to minor injuries as quickly, allowing damage to worsen before it’s noticed. Lines bred for the most extreme features, with very large round eyes, domed skulls, and virtually no nose, tend to have the most severe problems, including chronic tear overflow onto the face, corneal ulcers, and dark pigmented patches on the cornea from ongoing inflammation.
How to Tell the Difference
Context is everything. A cat that slow-blinks at you while lounging on the couch, then opens their eyes fully and goes about their business, is communicating contentment. A cat that holds one eye partially shut for extended periods, paws at their face, has visible discharge or cloudiness, or flinches away from light is showing signs of a medical problem.
Pay attention to duration, symmetry, and accompanying signs. Comfortable squinting is brief, involves both eyes equally, and happens in relaxed settings. Pain-related squinting tends to be sustained, often affects one eye more than the other, and comes with changes in behavior like hiding, reduced appetite, or avoiding being touched around the head. The more of these additional signs you see, the more likely something physical is going on.