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.