A cat whose eyelids look half closed is usually showing one of two things: the outer eyelids are drooping (squinting), or a translucent inner membrane is sliding partway across the eye. Both are signs that something is off, whether it’s pain, illness, or a specific eye condition. In a healthy, alert cat, the eyes should be fully open with no visible membrane covering the surface.
The Third Eyelid: What You’re Probably Seeing
Cats have a structure called the third eyelid, a translucent or whitish fold of tissue tucked in the inner corner of each eye. It normally stays hidden, sweeping across the eyeball only briefly to keep the cornea moist and clean. The membrane contains a T-shaped piece of cartilage surrounded by glands that produce part of the eye’s tear film.
When this membrane becomes visible, creeping partway across the eye from the nose side, it gives that characteristic “half closed” look. Many cat owners describe it as a white or bluish film covering part of the eye. If you’re seeing this in both eyes at once, or just one, the causes are quite different, and that distinction matters.
Sleepiness and Relaxation
The most benign explanation is that your cat is simply drowsy. When cats are falling asleep or just waking up, the muscles that hold the third eyelid in place relax, letting it drift across the eye. You’ll also see the outer eyelids droop to a half-closed position. This is completely normal and resolves within seconds to minutes as your cat fully wakes. If the half-closed look disappears once your cat is alert and moving around, there’s nothing to worry about.
Pain Is the Most Common Cause of Squinting
When cats hurt, their faces change in subtle but readable ways. Veterinarians use a scoring system called the Feline Grimace Scale that evaluates five features: ear position, eye tightening, muzzle tension, whisker shape, and head position. Eye tightening, where the cat narrows or partially closes its eyes, is one of the most reliable indicators of pain. It doesn’t have to be eye pain specifically. Cats with dental problems, abdominal discomfort, or injuries anywhere on the body will often squint.
If your cat’s eyelids look half closed and the cat also seems withdrawn, is eating less, or has flattened ears and tense whiskers, pain is a strong possibility.
Eye Injuries and Corneal Ulcers
A scratch on the surface of the eye (a corneal ulcer) is one of the most common reasons a cat will squint or hold one eye partially shut. Signs include inflammation around the eye, discharge, cloudiness on the eye’s surface, and sensitivity to light. An affected cat may paw at the eye or seem to have trouble seeing. Corneal ulcers can come from scratches during play or fighting, contact with rough surfaces, or as a complication of infections. They’re painful and can worsen quickly, so a cat holding one eye noticeably more closed than the other deserves prompt attention.
Upper Respiratory Infections
Feline herpesvirus is a leading cause of eye problems in cats, especially kittens. The virus targets the cells lining the conjunctiva (the pink tissue around the eye) and the cornea itself. Within two to six days of infection, it causes rapid cell damage that leads to conjunctivitis: red, swollen eyes with watery discharge that often progresses to thicker mucus. Both eyes are usually affected.
In the acute phase, you’ll typically see squinting, watery eyes, sneezing, and nasal congestion. The discharge often starts clear and watery, then shifts to yellow or green as secondary bacterial infection sets in. In kittens infected before their eyes have opened, pus can build up behind the sealed eyelids and, in severe cases, cause permanent damage.
The tricky part with herpesvirus is that it never fully leaves the body. After the initial infection clears, the virus hides in nerve cells and can reactivate during stress, causing recurring bouts of eye inflammation. Researchers have found viral DNA in the conjunctiva and eyelid tissue during chronic flare-ups, which helps explain why some cats deal with on-and-off eye issues for life.
Haw’s Syndrome: Both Eyes at Once
If the third eyelid is visibly protruding in both eyes simultaneously and your cat is otherwise fairly alert, Haw’s syndrome is a likely culprit. This condition causes bilateral protrusion of the third eyelids along with mild drooping of the upper lids. It often shows up alongside digestive symptoms like diarrhea.
The connection between gut problems and eye changes might seem strange, but research points to disruption of the gut-brain axis. Gastrointestinal pathogens, including certain bacteria and parasites, appear to alter the gut’s microbial balance in ways that affect the nerves controlling the third eyelid. Studies have found that cats with Haw’s syndrome sometimes have overgrowth of harmful bacteria in their intestines.
The good news is that Haw’s syndrome typically resolves on its own. In one case series, digestive symptoms cleared within 4 to 14 days, and the eye changes resolved in all cats within 11 to 39 days. A prebiotic-enriched diet designed for gastrointestinal health appeared to help, likely by restoring a healthier bacterial balance in the gut. The eye changes did not recur over follow-up periods of nearly a year or longer.
Horner’s Syndrome
If only one eye looks affected and you notice a combination of specific changes, Horner’s syndrome could be the cause. This condition results from damage or disruption to the nerve pathway that controls certain muscles around the eye. The classic signs are a constricted pupil, a drooping upper eyelid, the eye appearing to sink slightly into the socket, and protrusion of the third eyelid. You might notice the pupils are different sizes, which is most obvious in dim lighting because the affected eye can’t dilate normally.
What happens mechanically is that the muscles responsible for keeping the eye positioned forward in the socket lose their nerve supply. Without that input, opposing muscles pull the eyeball slightly backward, and as the eye retracts, the third eyelid passively slides forward to fill the space. Horner’s syndrome can result from ear infections, injuries, tumors, or sometimes has no identifiable cause.
Dehydration
When a cat is significantly dehydrated, the fat pad behind the eye shrinks as the body loses fluid. This causes the eyeball to sink slightly into the socket, a process similar to what happens in Horner’s syndrome but affecting both eyes. As the eyes recede, the third eyelids passively slide forward, giving that half-covered appearance. If your cat has been vomiting, has diarrhea, or hasn’t been drinking, dehydration could explain the eye changes. Other signs include dry gums, skin that stays tented when gently pinched, and lethargy.
What the Discharge Tells You
Pay attention to any fluid coming from the eye, because its appearance is a useful clue. Clear, watery discharge suggests irritation, early viral infection, or allergies. Yellow or green discharge that’s thick and mucus-like points to bacterial infection, either as a primary problem or layered on top of a viral one. Bloody discharge signals possible trauma or a more serious condition. If the discharge starts clear and transitions to colored and thick over a day or two, that progression typically means an initial irritant has led to infection.
Signs That Need Urgent Attention
Some combinations of symptoms signal a genuine emergency. A bulging eye, an eye that won’t open at all, or heavy colored discharge warrants same-day veterinary evaluation. Cloudiness or color changes in the eye itself, visible damage to the eye surface, and sudden apparent vision loss (bumping into furniture, misjudging jumps) are also red flags. Any sudden change in the eye’s appearance paired with obvious pain, such as constant pawing at the face, excessive blinking, or vocalizing, shouldn’t wait.
A single episode of visible third eyelids in a cat that just woke up is nothing. Third eyelids that stay visible for hours in an alert cat, squinting that persists through the day, or any one-sided eye change that doesn’t match the other eye are all worth investigating. The underlying causes range from self-limiting conditions like Haw’s syndrome to problems that can threaten your cat’s vision if left untreated.