The cat's belly rises and falls very frequently while sleeping.
The frequency of a cat's belly rising and falling is related to its breathing. A rapid rise and fall of a cat's belly indicates that its breathing is too rapid, which may indicate a problem with the cat's respiratory system.
Common problems include shortness of breath caused by allergies and respiratory infections, which require examination at the hospital for diagnosis. 
It's also possible that the cat has pleural effusion or ascites. The accumulated fluid can compress the chest cavity and lungs, causing the cat to have great difficulty breathing. In this case, the owner should take the cat to the vet for examination and treatment.
Cardiovascular abnormalities, such as hypertrophic cardiomyopathy, can also cause breathing difficulties in cats. Owners can observe whether the cat's tongue is purple or white, and whether it often breathes with its mouth open or coughs.
