If You Think Big Animals Always Sleep Standing Up, These Photos Will Set You Straight

If you thought that cows, horses, and other big animals always sleep standing up, you may need to spend more time down on the farm -- or just keep reading.

(Scroll down for photos.)

While some four-legged herbivores can doze lightly on their feet, they have to lie down to enter REM (rapid eye movement) sleep, as National Geographic pointed out in a recent article. That's the sleep phase in which dreams occur and when most muscles in the body are temporarily paralyzed .

Horses can stand in non-REM sleep without tipping over "because they have a protuberance at the end of their femur... this supports the hindquarters," Dr. Adrian Morrison, professor of behavioral neuroscience at the University of Pennsylvania's School of Veterinary Medicine , told The Huffington Post in an email. "To enter REM sleep, however, the horse must lie down because we are all paralyzed in that state."

So why is it so rare for us to see big animals lying down to snooze? Surprisingly, some big animals need little REM sleep.

"Contrary to some early ideas, sleep is not linked to animal size," Dr. Jerry Siegel, professor of psychiatry and biobehavioral sciences at the UCLA Center for Sleep Research , told The Huffington Post in an email. "Some big animals, such as lions, sleep a lot. Others, such as elephants, sleep very little. A related finding is that sleep does not appear to be related to brain size or cognitive capabilities."

Of course, many large animals -- especially those in a herd -- avoid lying down often in order to keep an eye out for predators.

Check out the photos below to see what it looks like when these large animals do fall into a deep sleep. Enjoy!

Why do we sleep and even dream? Check out the "Talk Nerdy To Me" episode below to learn more.