Also called the Flehmen response, when your cat opens its mouth and sticks out its tongue, it collects pheromones on its tongue. These pheromones give your cat important information about other cats in the area, including whether they are in heat.
If you have a juvenile kitten, she may stick her tongue out as part of her flehmen reaction. This is an open-mouthed smelling behavior that is an element of animal romance. Your cat may make a face that looks “gross” or disgusted to you, but in fact she smells other cats…especially female cats!
The reason your cat is falling asleep with his tongue out could be due to a few things: they are so relaxed they don’t realize it, they have problems with their mouth or breathing, or if they are still not quite asleep, they could use their Flehman response.
Normal panting in cats
Cats breathe rapidly when they pant, usually with their tongue out. Cats normally breathe through their nose, which means normal panting is usually due to one of the following: excitement or exercise. birth.