Do Animals Dream?

Rats have offered scientists some of the clearest windows into nonhuman animal dreaming. When snoozing, these tiny mammals follow a sleep cycle that’s very similar to that of humans. Like us, rats cycle through two different sleep stages, known as rapid-eye movement (REM) and slow-wave, or non-REM, sleep. People dream during both stages, with the most strange and vivid dreams usually occurring during REM sleep.

Human dreams can involve replaying events that occurred when we were awake. Matthew Wilson, a biologist who studies memory and sleep at the Massachusetts Institute of Technology, wanted to see whether rats drew from their experiences to create dreams too. In 2001, researchers in his lab trained rats to run along a circular track, rewarding them with food when they completed the task. They recorded the rats’ brain activity during these runs. Then they did the same while the rats were asleep.

The researchers compared the data and discovered something groundbreaking: The rats were rerunning the course in their dreams! When the rats ran in real life, neurons in their brain fired in a distinct pattern. As they slept, their neurons activated in the same sequence—as if the rats were replaying the memory of the course. This was one of the first experiments to demonstrate that animals other than humans have complex dreams during REM sleep. “Sleep and dreams are foundational functions of the human brain, and that’s something we share with animals,” says Wilson.

Scientists get some of the clearest looks into nonhuman animal dreaming from rats. The sleep cycles of rats and humans are very similar. Like us, these tiny mammals cycle through two different sleep stages. They’re called rapid-eye movement (REM) and slow-wave, or non-REM, sleep. People dream during both stages. But the most strange and vivid dreams usually happen during REM sleep. 

Human dreams may replay events that happened when we were awake. Do rats use their experiences to create dreams too? Matthew Wilson wanted to find out. He’s a biologist who studies memory and sleep at the Massachusetts Institute of Technology. In 2001, researchers in his lab trained rats to run along a circular track. The rats got a food reward at the end of the task. The researchers recorded the rats’ brain activity during these runs. Then they recorded it while the rats were asleep.

The researchers compared the data and made a groundbreaking discovery. The rats were rerunning the course in their dreams! When the rats ran in real life, neurons in their brain fired in a clear pattern. As they slept, their neurons fired in the same order. The rats seemed to be replaying the memory of the course. Humans have complex dreams during REM sleep. This was one of the first experiments to show that other animals do too. “Sleep and dreams are foundational functions of the human brain, and that’s something we share with animals,” says Wilson.

Birds aren’t born knowing how to sing. They pick up songs from other members of their species and then learn to sing them by practicing. Studies suggest that at least one bird, the zebra finch, likely rehearses its songs while sleeping. 

In 2017, researchers from the University of Utah found that when zebra finches fall asleep, their brains’ neurons fire in a specific pattern. The scientists realized that this pattern is the same one that happens when the birds are awake and practicing singing. “You can map it literally neuron to neuron, and musical note to musical note,” says David M. Peña-Guzmán, a philosopher at San Francisco State University in California who studies how animals experience the world.

As the sleeping birds dream of singing, the part of their brain that specializes in perceiving sound is activated. This means the animal’s brain is responding as if it’s hearing something, explains Peña-Guzmán. “Of course, the birds are not actually producing sound, but that’s the nature of a dream, right?” he says. “We experience things that are not really happening.”

Birds don’t know how to sing when they hatch. They pick up songs from other members of their species. Then they practice the songs to learn them. Studies suggest that at least one bird practices its songs during sleep. It’s the zebra finch.

Researchers from the University of Utah made a discovery about zebra finches in 2017. When the birds fall asleep, their brains’ neurons fire in a certain pattern. The scientists recognized this pattern. It also happens when the birds are awake and practicing songs. “You can map it literally neuron to neuron, and musical note to musical note,” says David M. Peña-Guzmán. He’s a philosopher at San Francisco State University in California, and he studies how animals experience the world.

Part of a bird’s brain specializes in detecting sound. That part activates as a sleeping bird dreams of singing. This means the animal’s brain acts as if it’s hearing something, explains Peña-Guzmán. “Of course, the birds are not actually producing sound, but that’s the nature of a dream, right?” he says. “We experience things that are not really happening.”

Fish evolved half-a-billion years before humans did. Studying fish can help scientists better understand the origins of sleep—and possibly the origins of dreaming. But how are scientists supposed to study sleep in an animal that doesn’t even close its eyes? 

To learn about mammals’ and birds’ sleep, scientists monitor their brains’ electrical activity by attaching sensors, called electrodes, to their heads. But it’s hard to attach electrodes to small animals, like fish or invertebrates—animals without backbones—such as bugs and crustaceans. So scientists typically use specific behaviors to identify sleep in these animals: things like lack of movement, reduced responsiveness, or a specific posture. But in 2019, Mourrain’s lab at Stanford came up with a way to investigate the brain patterns of zebrafish as they sleep.

The scientists built a device that allowed them to image all of the cells in the brain and body of young zebrafish. This meant that the researchers could observe changes in muscle tone, breathing, and heartbeat as the fish fell asleep. More important, they were able to observe individual neurons and synapse activity during rest. They found that, down to the cellular level, fish experience the same physical changes as mammals and birds do when they sleep—including REM-like sleep patterns. This suggests that REM and non-REM sleep states may have emerged in a shared ancestor more than 450 million years ago.

“There is no such thing as an animal that doesn’t sleep,” says Mourrain. Even simple animals, like fruit flies, cockroaches, and jellyfish, experience their environments and learn from them. All of that information needs to be integrated, and sleep does that, says Mourrain. So “it’s not completely crazy to think that a fish, worm, or even a fly could dream.”

Fish evolved half a billion years before humans did. Studying fish can help scientists better understand the origins of sleep. That might help them learn about the origins of dreaming. But fish don’t even close their eyes. So how can scientists study their sleep? To learn about mammals’ and birds’ sleep, scientists observe their brains’ electrical activity. They attach sensors, called electrodes, to their heads. But it’s hard to attach electrodes to small animals, like fish, or to invertebrates. These animals without backbones include bugs and crustaceans. To see if they’re asleep, scientists usually look for certain behaviors. The animal might not be moving. It might respond less or stay in a certain position. But in 2019, Mourrain’s lab at Stanford came up with an idea. It allowed them to study the brain patterns of sleeping zebrafish.

The scientists built a device. With it, they could image all the brain and body cells of young zebrafish. This meant that the researchers could observe changes as the fish fell asleep. They could watch muscle tone, breathing, and heartbeat. More important, they could observe individual neurons and synapse activity during rest. They found that fish experience the same physical changes as mammals and birds during sleep, even on the cellular level. Fish also have REM-like sleep patterns. This suggests that REM and non-REM sleep states may have emerged in a shared ancestor more than 450 million years ago.

“There is no such thing as an animal that doesn’t sleep,” says Mourrain. Even simple animals experience and learn from their environments. Fruit flies, cockroaches, and jellyfish are some examples. All that information needs to be processed, and sleep does that, says Mourrain. So “it’s not completely crazy to think that a fish, worm, or even a fly could dream.”