Why do kittens curl their bodies into a ball when they sleep

Cats sleep by curling their bodies into a ball so that the body has the smallest area for heat dissipation, which helps keep them warm and they are not too cold. In the process of continuous evolution, cats gradually formed this habit to better adapt to the natural environment. Especially in winter time, cats prefer to stay in a huddled position so that their body temperature can go up more easily. Sometimes cats sleep in a huddled position, perhaps because they are distrustful of their owners, or because they have just changed to an environment they are unfamiliar with. Such a posture is a defensive posture for unfamiliar environments and strangers, and when shrinking themselves into a ball, the cat will feel safe.