It's completely normal for a cat to sleep 12 to 16 hours a day, and some cats โ especially kittens and seniors โ sleep even more. Cats are crepuscular, meaning they're naturally most active at dawn and dusk, with long stretches of rest in between.
This sleep pattern is a holdover from their wild ancestors, who needed to conserve energy between short bursts of hunting. True deep sleep only makes up a portion of that time; much of a cat's rest is light dozing, where they can wake instantly if something interesting happens.
If your cat's sleep habits change dramatically โ sleeping far more or less than usual, or seeming lethargic during normal awake hours โ it's worth mentioning to your vet, since changes in energy can sometimes point to an underlying health issue.