Cats sleep for about 16 hrs a day on average, although it varies according to the individual – and it also tends to decrease as cats get older, as the sleep can become more fragmented and is a lighter, less deep form of sleep. Kittens (like puppies) tend to sleep for longer, as they’re still developing.
"As we tend to be more aware of them sleeping during the day – because we’re more likely to see them doing it – we may assume they sleep more often than they actually do."
Cats have evolved as a species that has a number of, short bursts of activity (such as to hunt and predate), followed by periods of rest to prepare for the next bouts of activity. If you look at a feral cat, for instance, they would spend a large chunk of their time hunting to obtain food – sometimes they will hunt unsuccessfully – so they’re expending time and energy but not always getting a calorific return. Whereas obviously with cats in domestic homes, we care and provide for them, including providing food. Although domestic cats are still motivated to hunt and predate, they don’t typically spend as much time doing it as evolution has prepared them for – so that tends to mean that domestic cats compensate by spending more of their daily time sleeping as opposed to being engaged in lots of physical hunting activity.
https://www.youtube.com/embed/VPh2m9l2DvU?wmode=opaque
So cats are good at conserving energy for short bursts of intense activity. One of the reasons why we believe they seek out warm spots to snooze and relax in is because they conserve energy more efficiently when they’re warm (hence the cat’s predilection for airing cupboards and duvets).
For about 9 to 12 hours a day, we think cats tend to engage in quite a light sleep when they’re probably still quite alert to any noises or movement around them that might indicate danger, a potential threat or possibly a source of prey around them. Also, cats are crepuscular, being more active at dawn and dusk as this is when their preferred prey is more likely to be active. That’s probably the main reason why we believe cats can see best in the low light conditions associated with these times of day. So a cat may be quite active while its owner is asleep, particularly in the early hours of the morning. As we tend to be more aware of them sleeping during the day (because we’re more likely to see them doing it), we may assume they sleep more often than they actually do.