I’m sure that you can answer the question without much thought. Common sense dictates that feral cats do not sleep as much as domestic cats. The reason: they have to survive on their wits. They have to be alert. They have to find their food (non-TNR cats!). There is less time to relax and sleep. In contrast, domestic cats in good homes are well cared for, they are fed, they are kept warm, they have security provided at all times. In a good home they have rhythms and routines. They are reassured and aren’t anxious. Therefore they can sleep contentedly. If they are confined to the indoors-full-time, it is likely that they will not have enough stimulation. In order to cope with this they snooze and they snooze for longer hours than domestic cats that are allowed outside. They are killing time.

RELATED: Do cats sleep more in winter?
Snoozing is a natural reaction in the domestic cat to a reduced need to be active. And the need to be active arises in the need to hunt for food. However, I think it’s a myth to say that domestic cats sleep for 18 hours a day as many websites state. It simply isn’t true. Cats who are extensively sleeping are actually snoozing. They are alert. This is also inherent in domestic cat behaviour. It is their survival instinct which is switched on 24/7.
A domestic cat who is meant to be sleeping wakes up in about one second. Their ears will respond to sounds despite the fact that they are meant to be sleeping. This tells me that they are not sleeping. They are snoozing. They are resting. You can tell when a cat genuinely sleeps because they dream. Their whiskers twitch and their legs jerk sometimes. Sometimes they make sounds just like people.
I’ve answered the question in the title. There is a subtext to this post. It’s an area of cat caregiving which concerns me. There is a movement towards full-time indoor cats. I read on one website that around 40% of domestic cats in America are confined to the home. That’s great but there has to be a caveat. Full-time indoor cats are removed from outdoor stimulation. They are much safer. Wildlife is much safer. But they are put into a position where they are predisposed to acute boredom through lack of stimulation. They lie around all day. Perhaps their owner thinks that they are happy to snooze and sleep all day because cats do that kind of thing. They think it’s no problem to not stimulate their cat because they like to sleep.
This, in my view, simply isn’t true. If full-time indoor cats are forced to sleep for long hours, much longer than normal, is a survival instinct. It is a survival instinct in the opposite direction to the one that feral cats have to activate. Domestic cats shut down and kill time but they would much rather be in compliance with their natural instincts which is to hunt, stalk prey, spend hours doing that and then eat and then groom themselves and sleep. And so the cycle restarts.
If you can achieve that within a full-time indoor environment you reach the pinnacle of cat caregiving and ownership.