The reason why cats sleep on their backs.
The cat is comfortable in its surroundings and knows there's nothing that could harm it, so it sleeps on its back. This is a sign that it's relaxed and comfortable, indicating that it feels attached to and belongs to its home and its surroundings, considering it its territory. If the owner notices the cat sleeping on its back and doesn't mind, this is not unusual and will not harm the cat. 
How many hours a day do cats sleep?
Cats typically sleep 15 hours a day, and kittens around 20 hours, but the actual deep sleep time is only about four or five hours. Adult cats often sleep in a semi-conscious state, alternating between deep and light sleep. As predators, this semi-conscious state helps cats maintain sufficient energy while also allowing them to observe their surroundings.
