You can learn a lot about animals just by the way they sleep.

Many large animals can sleep lightly while standing, thanks to an anatomic bonus called the stay apparatus. This interconnection of tendons and ligaments allows prey animals to stand for long stretches while expending very little effort — and while staying on alert for predators. But to get deep, revitalizing sleep, even large animals have to lie down.

The more vulnerable the prey, the less time is allotted to sleeping. Believe it or not, giraffes do fine with only about five minutes of deep sleep per day. Elephants get about three hours of shut-eye daily, while rhinos sleep a very human-like eight hours each day.

You may not think of your couch kitty or pampered pooch as predators, but watching them sprawled out in deep sleep tells you all you need to know.