You've probably wondered if animals dream if you've ever watched a dog nap.

It's a difficult question. We still don't know why humans dream or why dreams are important. It's even more challenging to study animal dreams because dogs can't tell us what made them whine or run during snooze.

Depending on how they are defined, animal dreams may have intriguing implications. (Learn more about sleep secrets.)

"I think dreaming allows us to extend several cognitive capacities to animals, such as emotion, memory, and even imagination," says David M. Pea-Guzmán, a philosophy of science professor at San Francisco State University and author of 
We know primates have emotions, but consider spiders, which, according to a recent study, may have a REM-like sleep and even visual dreams. Spider dreams sound ridiculous, but they could be true.

"We have this idea of dreams being a confabulatory narrative with crazy, vivid elements," says MIT neurobiologist Matthew Wilson. "However, when we look at animal models, we're simply trying to understand what happens during sleep that may influence learning, memory, and behaviour."

What do cats fantasise about?

Domestic cats were among the first animals studied in terms of dreams. Michel Jouvet, a pioneer in sleep studies, discovered evidence of feline dreaming in the 1960s when he observed cats' sleeping behaviour and dramatically altered it.

Despite the intense mental activity that drives our dreams, human muscles do not move much during REM sleep. This state of atonia ensures that our bodies do not act out our fantasies, no matter how real they appear. Jouvet discovered that a brainstem structure called the pons seemed to regulate REM sleep and cause partial paralysis in cats.

Jouvet, on the other hand, caused a dramatic change in behaviour by removing parts of the pons. The cats began to move as if awake, hunting, jumping, grooming, and aggressively defending themselves against invisible threats while their brains were deep in REM sleep.

Jouvet referred to this as paradoxical sleep because fully active. This provided insight into what was going on in the sleeping brains of the cats.

"The cats displayed behaviours that are very easy to interpret as mapping onto a waking experience," Pea-Guzmán says.

Animals dream too—here's what we know - National Geographic https://t.co/IbxJMkxi1g

— ChiChi Greenblatt (@ChiChiGreenblat) November 3, 2022

Rats remember maze memories.
According to research, rats can rerun a maze while asleep after running it during the day. The hippocampus, a part of the brain responsible for making and storing memories, remembers the neuron pattern of navigating the maze when the rat is awake. Later, while sleeping, the brain reproduces the same way, implying that the rat repeatedly remembers or relearns the labyrinth.

This discovery from 2001 was among the first to suggest that animals had complex dreams. And it was only the beginning, according to co-author Wilson, an MIT neurobiologist.

"We've done other studies that suggest how memories of previous experiences are brought up again in sleep may be similar to what we would experience as dreams."

According to the rat brain studies, when maze memories occur during sleep, the visual imagery associated with them is also reactivated, implying that the sleeping rodents saw what they had seen in the maze while awake. The same has been discovered for auditory and even emotional areas that reactivate during REM sleep when the rat reruns the labyrinth. (Find out why rats avoid hurting other rats.)

"A lot of things point to a significant re-experiencing of the awake state during sleep," Wilson says. "If we want to call it dreams, I'm excellent with that description." What's intriguing is, if that's what's going on, what does it all mean?"

Zebra finches remember songs.

Despite their reputation for lyrical songs, zebra finches are not natural singers. The birds must learn by listening, practicing, and perhaps dreaming.

Researchers discovered in 2000 that neurons in birds' forebrains fire in a specific pattern when they sing a song, which scientists can recreate note for note. The birds' brains replicate this same pattern while they sleep, repeating the music they heard and sang that day, implying that they remember and practice songs while they sleep.

The researchers believe the songbirds fantasise about singing. Do sleeping birds relive their waking lives? Or are singing dreams more akin to algorithms that operate without conscious awareness? Scientists may be getting closer to discovering the answer.

After two decades of research, finches were discovered to have a similar sleep structure to humans, including REM sleep. More recent research indicates that the birds move their vocal muscles to match the music in their brains and that they can be prompted to sing a song played to them in their sleep.

Sleeping finches also produce song variations, implying that they gather sensory information while awake and adapt by improvising new versions to promote learning in a dreamlike state.

According to Stanford neurobiologist Philippe Mourrain, Zebrafish experience REM-like sleep while sleeping deeply with the fish. These fish lose muscle tone, develop arrhythmic heartbeats, and exhibit brain activity similar to that of an awake fish while sleeping. One notable difference between humans and other animals was that the fish did not move their eyes. (They also did not close their eyes because they lacked eyelids.)

The discovery implies that REM sleep, the state in which most dreams occur, evolved at least 450 million years ago—before land and aquatic animals diverged in their evolution.
"Twenty years ago, people told me that fish don't even sleep," Mourrain says. "Now we see... insects, spiders, and vertebrates share those behavioral characteristics." You also lose control of your most critical regulatory systems in REM sleep. Evolution would not have preserved such a fragile state if it didn't matter."

But why is dreaming important? Does the fact that REM sleep has been conserved throughout evolution imply that even fish can desire it? (Read about how REM sleep can be used as "overnight therapy" in humans.)

That depends on how you define dreaming. Dreaming, according to Mourrain, is best described as a simple shuffling of synapses or a reset of the neural connections that prepares our nervous system for the coming day through processes such as memory consolidation and cognition optimization.

"I wouldn't be surprised if actual dreams were discovered in animals, and I believe we'll be able to demonstrate that scientifically at some point," he says.

"During the day, you did something, and your brain will replay it, integrate it, and mix it with other experiences." We are not the only species that can remember and learn."