Cats are predators they hunt for food and hunting takes a lot energy. The fact our house at is in average spoiled and doesn't hunt didn't make evolution nor mother nature take away this life saving behaviour.
The fact they can sleep under strange circumstance is due to their flexible skelet, although it isn't said cats do not have health issues and hernia.
Cats see a mouse and start playing together hahaha, modern cats don't want to hunt, they just want to play and sleep, sleep and sleep.
I've seen some cats hanging around some birds.