Why Animals Dream

When a sleeping dog twitches its paws and makes small sounds, you have probably wondered if it is dreaming. The answer, based on the best available neuroscience, is almost certainly yes.

Dreaming is not uniquely human. It is something the brain does across an enormous range of species, and understanding why sheds real light on what dreams are actually for.

The Signature of Dreaming in the Brain

To understand animal dreaming, start with the brain signature that dreaming produces.

During REM sleep, the brain generates a distinctive pattern: the cortex becomes highly active in a way that closely resembles waking, while the body is largely paralyzed. Eye movements occur. Heart rate and breathing become irregular. These are the same features observed in humans during vivid dreaming.

REM sleep has been documented in nearly all mammals studied. Dogs, cats, rats, mice, primates, elephants, dolphins. The pattern is consistent enough that researchers treat REM sleep as the physiological correlate of dreaming. When a dog's legs twitch and its eyes move beneath closed lids, its brain is producing the same cortical activation pattern that a dreaming human brain produces.

This is not coincidence. Dreaming, in some form, appears to be a feature of having a mammalian brain.

What Rats Actually Dream About

The most direct evidence that animals dream about specific waking experiences comes from a landmark 2001 study at MIT's Picower Institute for Learning and Memory.

Neuroscientists Kenway Louie and Matthew Wilson trained rats to run through a maze, then monitored their hippocampal activity throughout the task. The hippocampus contains place cells that fire in specific sequences as an animal moves through space, producing a kind of neural map of the journey.

When the rats entered REM sleep, Louie and Wilson found that the place cells fired again in the same sequences they had produced during the maze run. The match was precise enough that the researchers could reconstruct where in the maze the rat had been based on its sleeping brain activity alone.

The rats were replaying their waking experience during sleep. In any reasonable interpretation, they were dreaming about the maze.

This study, published in the journal Neuron, was the first to provide direct neural evidence that animals re-experience specific waking episodes during REM sleep. It also gave early support to the memory consolidation theory of dreaming: that the sleeping brain replays recent experience in order to strengthen and organize it.

Birds Rehearse Their Songs While Asleep

Mammals are not the only animals with evidence of dream-like states. Songbirds provide an equally striking case.

In research published in Science in 2000, Amish Dave and Daniel Margoliash at the University of Chicago studied zebra finches, a species known for learning complex songs. The researchers recorded the firing patterns of neurons in the premotor area of the finch brain during active singing, then monitored those same neurons during sleep.

The result: the neurons fired during sleep in sequences that closely matched the patterns produced while the bird was singing. The finch's sleeping brain was running through the song in sequence, as though rehearsing it.

The connection to learning is direct. Zebra finches acquire their songs during a critical developmental period, and sleep is understood to play a key role in stabilizing that learning. The sleep rehearsal appears to be how the song gets locked in.

The parallel to human skill consolidation during sleep is difficult to ignore. Humans learning a musical instrument, a motor skill, or a language also consolidate that learning during sleep. The mechanism appears to reach back to an ancient feature of the vertebrate brain.

How Far Down Does Dreaming Go?

Mammals and birds sit on different branches of the vertebrate family tree, both sharing the REM-like sleep state. The question is how deep the pattern goes.

In 2016, a research team led by neurobiologist Gilles Laurent at the Max Planck Institute for Brain Research published a study in Science examining Australian bearded dragons. The reptiles showed clear alternation between two distinct sleep states: one resembling slow-wave sleep and one resembling REM, complete with brain wave patterns, eye movements, and periodic activation. Reptiles had long been assumed to lack REM sleep entirely. They appear not to.

The study pushed the evolutionary origin of REM-like sleep back further than previously thought. If reptiles, mammals, and birds all show versions of the same pattern, the common ancestor predates the divergence of those lineages, which goes back more than 300 million years.

More recently, researchers have identified what appear to be sleep states in even more distant relatives. A 2022 study by University of Konstanz researcher Daniela Rößler observed jumping spiders during rest and documented periodic eye movements, leg twitching, and curling behavior consistent with an REM-like state. The spiders appeared to cycle through these episodes repeatedly throughout the night.

Whether spiders dream in any subjective sense is a genuinely open question. But the physiological pattern is there.

The Evolutionary Logic

Why would dreaming be conserved across so many species over hundreds of millions of years? Evolutionary biology offers a useful heuristic: if something persists that widely for that long, it almost certainly does something useful.

The leading theory is memory consolidation. During REM sleep, recent experiences are replayed and integrated with older memories. The connections that matter are strengthened; the noise is cleared away. This function would have been valuable from the earliest nervous systems forward: an animal that wakes up having consolidated what it learned yesterday has a meaningful advantage over one that does not.

A second theory is threat simulation. The dreaming brain tends toward negative and threat-relevant content across species. This may reflect a survival function: rehearsing threats and practicing responses during a safe period (sleep) prepares the animal for real threats while awake. Some researchers argue that nightmares in humans are an extreme version of this same ancient function.

A third is emotional processing. Stress hormones are reduced during REM sleep, and emotionally charged memories appear to be processed differently at night than during the day. The emotional weight of an experience may be partly metabolized during sleep, allowing waking behavior to proceed without being overwhelmed by accumulated stress.

These three theories are not mutually exclusive. Memory consolidation, threat rehearsal, and emotional regulation may all be part of what sleep accomplishes, producing different aspects of the dreaming experience.

What Animal Dreaming Tells Us About Our Own

The implications for how we understand human dreams are significant.

Dreaming is not a uniquely human capacity. It is not primarily about meaning in the symbolic or narrative sense. It is what brains do when they are offline processing recent experience. The content of dreams is drawn from what was emotionally and cognitively significant during the preceding waking period, and the sleeping brain replays, reorganizes, and integrates it.

This makes dreams less mysterious in one sense and more interesting in another. Less mysterious because the mechanism is biological, ancient, and shared across species. More interesting because the question shifts from "what does this symbol mean" to "what did my brain consider worth processing while I was unconscious?"

The rat's brain chose the maze. The finch's brain chose the song. What your brain chooses to replay may be more informative than any symbolic interpretation.

Questions Worth Sitting With

If your brain replays what it considers emotionally significant, what would you expect it to have processed last night?
Do you notice patterns in the subjects of your dreams that track what you were preoccupied with during the day?
Knowing that animals dream about specific experiences, does that change how you think about what your own dreams are made of?
What might it mean that dreaming has persisted across 300 million years of evolution?

One Thing to Try

After waking from a dream, before reaching for your phone, ask: what experience from yesterday might my brain have been processing? See if the dream content makes more sense from that angle.

One perspective among many. Dream science is an active and sometimes contested field. The studies described here represent significant findings, but the subjective experience of dreaming and its full significance remain areas of ongoing research.

Try describing a recent dream to the doz.ing dream interpreter to explore what it might be reflecting.