First and foremost, it's important to understand that cats are predators. In the wild, they would spend a large majority of their time hunting for food. This is an energy-intensive activity, and as such, cats need to spend a lot of time resting in order to conserve their energy reserves.
In captivity, however, cats don't need to worry about hunting for food as they're typically fed on a regular schedule. As a result, they have a lot more free time on their hands...which they choose to spend sleeping! In fact, studies have shown that the average house cat spends around 16 hours per day sleeping. That's nearly two-thirds of their day!
But why do they need to sleep so much? Well, part of the reason has to do with their biological makeup. Cats have very fast metabolisms and burn through energy quickly. They also have very small stomachs relative to their body size, which means that they need to eat small meals more often throughout the day in order to meet their energy needs. This frequent eating results in a lot of down time in between meals, which cats use as an opportunity to catch some Zs.
In addition to all of this, cats also tend to be lazy by nature. They are generally content lounging around and taking naps all day long. So if your cat is spending most of her time sleeping, there's nothing to worry about! She's just being a typical cat.