The life of a house cat basically consists of four activities: playing (perhaps patty-cake), ruining upholstered furniture, eating and sleeping — and that last activity takes up the most time by far.
Cats are among the top sleepers in the animal kingdom, and for good reason: They are, in the wild at least, predators, and their prey doesn’t usually want to get caught. This means that cats have to do quite a bit of chasing, and that chasing (hopefully) culminates in a huge burst of energy for the final takedown. If they don’t have the energy for a successful hunt, they don’t get to eat. So they’ve evolved to sleep (or sometimes just rest) for most of the time they don’t spend hunting, saving up the strength to catch dinner.
What they eat may contribute to their sleep time, as well. They live on protein, which packs a lot of energy into a relatively small package. They needn’t spend endless hours grazing or foraging the way large herbivores do.
So, just how much sleep does a wild cat get? Anywhere from 16 to 20 hours a day, typically. For very young and very old cats, it’s near the upper end of the range, and newborns sleep almost 24/7.
While cats do spend at least two-thirds of their lives asleep, they’re not “asleep” in quite the same way humans are. They do experience both non-REM and REM sleep, but for cats, “asleep” is not “off the clock.” Cats are always on the alert, even when they’re dozing. If a strange noise wakes them up, they’re almost instantly aware and fully operational. It’s an ability that cats (and wild animals in general) depend on to stay safe, and fed, in nature’s Darwinian existence.
House cats, of course, have left “survival of the fittest” behind. They needn’t chase down their dinner. They are, on the contrary, served their meals, sometimes gourmet ones, in a timely fashion. But the instincts haven’t changed; house cats have the same genetic programming as feral cats.
Domesticated felines, like their wild counterparts, sleep about 16 hours a day, on a pretty regular schedule, saving up their energy for the hunt. You never do know when the gourmet food’s going to run out.