A red ruffed lemur relaxes after a long day of being a lemur. If they are anything like the dozens of other mammals that have been studied, lemurs dream too.
Do animals dream? Signs point to "yes" for most animals, but exactly what they dream -- or why they do it -- is not so clear.
Take, for example, naked mole rats at the National Zoo. David Kessler, small mammal biologist at the Zoo, holds up one of the fleshy, pink, wrinkly and nearly blind rodents. He says when they sleep, they do something kind of funny.
"They grind their incisors, their front teeth. And you'll see them on their back, twitching a little bit, with those front teeth going back and forth, and back and forth," he says.
Could they be dreaming? Dreaming of burrowing and gnawing?
"I have no idea, you know, I can barely get in the mind of my wife or my son or myself so I don't presume to get in the mind of a blessed naked mole rat," Kessler says.
No one can, really. But we can measure brainwaves, rapid eye movement (REM) sleep.
"When we're in a deep sleep, our muscles don't move, but our eyes are moving, moving, moving," says Don Moore, associate director of Animal Care Science at the Zoo.
In humans, it's when we dream.
"We've shown REM sleep in a lot of different animals," Moore says. "The question is, What's going on in their brains?" he says.
Normally when you go into REM sleep and dream, your brain paralyzes your body so you don't actually get up and run around while you're dreaming.
Patrick McNamara, an associate professor of Neurology at the University of Boston School of Medicine, says that when you turn off that paralysis in animals "and the animal goes into REM sleep, they start to exhibit these behaviors that strongly suggest they're seeing things and they're reacting to what they're seeing. For example if it's a cat, it appears to be chasing a mouse...slapping it around..."
So, animals dream. They dream about being animals, doing the things that animals do. But why? Why do animals dream? Why do we dream?
"That's one of the fundamental mysteries of biology. Why is there REM sleep?" McNamara says.
To realize just how much of a mystery it is, you need to realize that REM sleep -- dreaming -- is kind of insane.
"Your brain gets very highly activated, even more activated than in it is during the daytime, but your body is paralyzed -- you can't move...and then you're forced to watch these things we call dreams. So why would Mother Nature do something like this? Nobody knows," he says.
But there are some clues, written in to evolutionary history. The most ancient animals -- the fish, the amphibians, the reptiles -- don't appear to dream. Birds do, but just for a few seconds at a time. But even the most primitive mammals seem to dream.
"It has something to do with the advent of mammals. And so mammals, in particular, needed this form of sleep," McNamara says.
It could be a product of advanced brains. It could be a way to sort through memories or reinforce learning. It could just be an evolutionary byproduct that we and most other species of mammal are stuck with.
It's something we do every night, and we don't know why.