Almost every page on this subject tells you a cat sleeps twelve to sixteen hours a day. We went looking for where that number comes from and could not find it in a single guideline, veterinary school page or published study. It appears to be repeated from site to site. Here is what can actually be sourced, and it is more useful anyway.
The Merck Veterinary Manual, in its section on cat behavior, gives a time budget rather than a sleep number: cats sleep 40 percent of the day, rest 22 percent of the day, and groom 15 percent. It credits that to a 1981 study of free-ranging farm cats.
Two things follow from it. Forty and twenty-two together mean roughly fifteen hours a day doing nothing much, of which actual sleep is the smaller part. And the cats measured were farm cats with the run of the place, not an indoor cat with a sofa, so treat it as a shape rather than a target.
Laboratory measurements disagree sharply. A sleep research database at Boston University records total daily sleep in cats as 5.6 hours, from work done on restrained animals wired to an EEG. That is measuring something different from a cat dozing on a windowsill. We are showing you both because the honest answer is that there is no agreed single number, and anyone who gives you one has not looked.
This one is well measured. A 2019 study tracked fourteen indoor cats around the clock and found their activity has two peaks, one in the morning before sunrise and one in the evening. The researchers concluded that this pattern, rather than individual chronotypes, is what characterises the species. The word for it is crepuscular. A cat tearing around at five in the morning is not broken.
Not how many hours. Whether the number changed.
Texas A&M's veterinary college puts the problem in one sentence.
Cats tend to hide their symptoms, which is probably due to survival instinct.
And what that looks like from the sofa:
Most signs of illness or injury are subtle, including sleeping more than normal; not getting up to greet you, if that is normal behavior; or laying and sleeping in the same position for long periods of time.
The senior care guidelines list increased sleeping not under laziness but under mobility, which is to say it can be a sign of pain. A cat with sore joints does not limp in front of you. It stops jumping, stops greeting you at the door, and sleeps in the one spot it can still get to.
A study of fifty cats diagnosed with osteoarthritis found the average age at diagnosis was twelve, and that the diagnosis leaned heavily on what owners had noticed rather than what turned up in the exam room. Its conclusion was that physical examination abnormalities may not be detected. You are the one who sees the change. The vet mostly sees a cat being brave in a strange room for ten minutes.
None of that is a diagnosis. It is the difference between a cat who is fine and a cat worth looking at, and it is the part you can see and a vet cannot.