Cats wink with one eye for two very different reasons: they’re communicating something positive, or something is bothering that eye. A quick, relaxed wink or slow blink is typically a sign of trust and contentment. But persistent squinting or holding one eye partially closed often signals pain, irritation, or infection. The difference between the two is usually easy to spot once you know what to look for.
The Slow Blink: A Cat’s Version of a Smile
When your cat looks at you with partially closed eyes and blinks slowly, it’s the feline equivalent of a warm, relaxed smile. Cats narrow their eyes this way when they feel safe and content, much like how human eyes naturally narrow during a genuine smile. Cat behaviorists interpret this as a deliberate signal of benign intentions, since cats generally view unbroken eye contact as a threat. By closing their eyes, even briefly, a cat is saying it trusts you enough to let its guard down.
A 2020 study published in Scientific Reports tested this formally. Researchers found that cats produced significantly more eye-narrowing and half-blinks when their owners slow-blinked at them compared to when no interaction occurred. In a second experiment, even an unfamiliar person could get the same response. Cats were also more likely to approach a stranger who slow-blinked at them than one who maintained a neutral expression. Both owners and strangers could start this exchange, suggesting it’s not just a learned response to a familiar face but a broader communication tool cats use with people in general.
You can try this yourself. Narrow your eyes in a relaxed way, then slowly close them for a couple of seconds. Many cats will mirror the expression back, creating a kind of back-and-forth conversation. It works best when your cat is already calm and settled, not mid-play or mid-meal.
How a Friendly Wink Looks Different From a Problem
A social wink or slow blink is brief, relaxed, and symmetrical in how the cat holds its face. The eye opens fully again afterward, and the cat goes on with its day looking comfortable. There’s no discharge, no redness, and no pawing at the face.
A medical squint looks different. The cat holds one eye partially or fully shut for extended periods. You might notice it repeating the squint over and over, or the affected eye may look different from the other one. Healthy cat eyes are bright, clear, and equal in pupil size, with no squinting from either eye. Any deviation from that pattern, especially in just one eye, points toward irritation or injury rather than communication.
Common Reasons for Persistent One-Eye Squinting
When a cat keeps winking or squinting with the same eye repeatedly, the most likely culprits are physical irritation, infection, or injury.
- Foreign objects: Grass, dust, sand, or even a stray eyelash can get trapped under the eyelid. The eye will typically produce a thin, watery discharge as it tries to flush the object out. If the tearing doesn’t clear it, the squinting will persist.
- Conjunctivitis: Bacterial or viral infections of the tissue lining the eye cause redness, swelling, and a mucus-like discharge that can be white, yellow, or green. Allergies to dust, pollen, mites, or fleas can trigger the same reaction.
- Corneal ulcers: A scratch or puncture to the clear surface of the eye causes squinting, light sensitivity, and sometimes a cloudy appearance. Cats with corneal ulcers often rub at the affected eye and may behave as though they’re having trouble seeing. Cornell University’s Feline Health Center notes that discharge from an injury typically appears only in the affected eye, which is one way to distinguish it from a systemic infection.
- Upper respiratory infections: These often affect the eyes alongside the nose and throat, producing sticky yellow or green discharge. If your cat is also sneezing or congested, this is a likely cause.
- Household irritants: Cleaning chemicals, cigarette smoke, perfumes, and aerosol sprays can all irritate a cat’s eyes and trigger squinting, especially if the cat was close to the source on one side of its face.
The Third Eyelid and What It Means
Cats have a translucent inner eyelid called the nictitating membrane that sweeps horizontally across the eye to protect and moisten it. You normally don’t see much of it. When it becomes visible as a white or pinkish film creeping across one eye, it usually means that eye is irritated, inflamed, or that the cat is unwell. A visible third eyelid in one eye, combined with squinting, is a reliable sign that something physical is going on rather than a friendly social signal.
What Eye Discharge Colors Tell You
The type of discharge accompanying a squint gives useful clues about the cause. Clear, watery tears usually point to a foreign body or mild irritation, something the eye is actively trying to wash away. White, yellow, or green mucus-like discharge suggests infection, whether bacterial, viral, or related to conjunctivitis. Thick or colored discharge from one eye specifically often indicates a scratch or puncture wound. If an upper respiratory infection is involved, you’ll typically see sticky yellow-green discharge from both eyes along with nasal symptoms.
Signs That Need Prompt Attention
A single brief squint with no other symptoms is rarely concerning. But certain signs suggest the problem is more serious. Watch for cloudiness or a color change in the eye, pupils that appear different sizes from each other, discharge that’s thick or colored, visible swelling, bleeding, or a cat that keeps pawing at its face. Light sensitivity, where the cat avoids bright rooms or turns away from windows, can indicate a corneal ulcer or deeper inflammation inside the eye. Eye problems in cats can escalate quickly, so persistent squinting that lasts more than a day or comes with any of these additional signs warrants a veterinary exam rather than a wait-and-see approach.