Cats sleeping in a ball may be caused by fear, cold, pain, etc. Cats will curl up into a ball to protect themselves in unfamiliar environments and increase their sense of security. This will also happen to cats if the temperature is low. It can reduce heat loss. Sometimes cats curl up into a ball to indicate physical discomfort. Owners should pay attention to observing the cat. If any abnormality occurs, the cat should be sent to the pet hospital for examination and treatment in time.