New cat's third eyelid showing when asleep

Question

I have a new rescue cat who is slowly settling in - she's very timid, but getting braver every day. This question is about her eyes.

When she is falling asleep/already asleep and her eyes are half-closed, her pupils kind of roll back into her head. I know this is because of the third eyelid cats have, but I keep seeing online that it could be a sign of serious illness. I've only ever seen it when she's falling asleep, and when she is deeply asleep her eyes are closed all the way and I don't see anything. I've never seen it happen when she is awake.

I'm mostly worried because I saw that it could be a symptom of dehydration, and I immediately started wondering if she was getting enough water. I've never seen her drink, and I haven't been monitoring the water level in her bowl closely enough, I guess. She eats a small can of wet food in addition to dry food every day, and there's urine in her litter box every day. I can't really speak to lethargy or not, because she mostly spends her time hiding or sleeping.

I'd take her to the vet, but since she's been here less than a week I can't really get close to her yet, so the effort of catching her and putting her in the carrier would be fairly traumatizing. I will probably call the vet in the morning regardless, to be on the safe side.

Has anyone seen this before? Do I just have a cat with weird sleep habits, or is it something worse?

Answer 1

I'm not a vet, so take this with a grain of salt.

I've seen similar effects on several cats and dogs. When they start falling asleep, the muscles relax one after another, not all at the same time. The nictitating membrane ("third eyelid") tends to close sooner than the external eyelid because the muscles of the eyelid relax later.

I've even seen my dog open his eyes during sleep (he was dreaming). His nictitating membrane was fully closed but his eyelids fluttered open. You could clearly see the moment he woke up because only then did his membranes open as well.

As long as you see the nictitating membrane only while your cat is sleeping or falling asleep, it seems fairly normal. Seeing the membranes when the cat is awake and active is not normal and you should consult a vet. Should your cat ever stop eating, drinking or using the litter box for a whole day, treat it as a medical emergency and consult a vet immediately.