Do Animals Dream? The Surprising Science of Animal Sleep States

Do you ever wonder what your dog or cat dreams about as they snooze the day away, legs twitching and paws paddling at nothing? For a long time, dreaming was assumed to be a uniquely human, or at most a uniquely mammalian, experience. New research is rapidly overturning that assumption. From spiders to octopuses, scientists are finding dream-like brain and body states across an astonishing range of species, and the discoveries are offering exciting new clues about why dreaming evolved in the first place.

Quick Facts

• Jumping spiders show REM-like eye movement and limb twitching, the first such evidence ever found in an arachnid
• Octopuses cycle between “quiet sleep” and a roughly 40-second “active sleep” stage marked by flashing skin colors
• Rats have been shown to replay the exact brain activity of a maze run while sleeping, hours after the fact
• REM-like sleep has now been documented in mammals, birds, reptiles, cephalopods, and at least one arachnid

The Jumping Spider Study

In 2022, behavioral ecologist Daniela Rößler and her team at the University of Konstanz and the Max Planck Institute of Animal Behavior published a study in the journal PNAS that surprised much of the sleep research community. Studying jumping spiders (Evarcha arcuata), the team noticed the tiny arachnids hung motionless at night, occasionally twitching their legs, curling them toward their bodies, and even twitching their silk-producing spinnerets, much like a dog or cat in the middle of a REM cycle.

The real breakthrough came from studying spiderlings, whose exoskeletons are translucent enough for researchers to see straight through to their retinal tubes. Using infrared cameras, the team filmed 34 young spiders overnight and documented regular, recurring bouts of rapid retinal movement, roughly analogous to the rapid eye movement that gives REM sleep its name, each lasting just over a minute and recurring periodically through the night. It was the first time this kind of REM-like state had ever been documented in any arachnid, or in fact in any invertebrate outside of cephalopods, according to the study published in PNAS.

Birds Dream Too

Birds offer some of the clearest evidence of dream-like brain activity outside of mammals. Studies on common pet birds such as budgies and parakeets found they physically reacted and vocalized in their sleep when earlier played recordings of other birds in apparent distress, almost as if replaying or responding to a remembered threat. Pigeons have gone a step further: researchers have recorded pigeons appearing to rehearse their songs while asleep, with brain activity patterns during sleep closely mirroring the patterns recorded while the birds sang while awake, a strong hint that the sleeping brain may be replaying and reinforcing the very skill it needs while conscious.

Octopuses and Cuttlefish

Even octopuses, despite being marine invertebrates with a brain structure wildly different from our own, show some of the most visually striking evidence of dream-like sleep found anywhere in the animal kingdom. A 2021 study led by neuroscientist Sylvia Medeiros at Brazil’s Federal University of Rio Grande do Norte found that octopuses cycle between two distinct sleep states. During “quiet sleep,” which typically lasts around seven minutes, the octopus stays still with pale, uniform skin. This is followed by a much shorter “active sleep” phase, lasting under a minute, during which the animal’s skin flickers rapidly through vivid colors and textures, its eyes move, and its arms and suckers twitch, closely resembling the displays octopuses use while awake and interacting with their environment, according to reporting by Smithsonian Magazine on the original study.

As study co-author Sidarta Ribeiro put it, it’s tempting to try to read an octopus’s dreams directly off its skin, though he’s careful to note that no one can say for certain yet what, if anything, the octopus is experiencing during those flickering bursts of color. Cuttlefish, close cousins of the octopus, show a strikingly similar quiet-to-active sleep cycle, suggesting this dream-like sleep state may be a shared trait across cephalopods generally.

What Do Dogs and Cats Actually Dream About?

For pet owners, the most familiar version of this question involves the family dog twitching and “running” in its sleep. Mammalian REM sleep has been studied far longer than any of the discoveries above, and one particularly famous piece of evidence comes from rats rather than dogs. In a widely cited study from MIT, neuroscientist Matthew Wilson and colleagues recorded the exact pattern of brain cell activity in a rat’s hippocampus as it ran through a maze. Remarkably, when the rat later fell asleep, the same sequence of brain activity replayed itself in the same order, as if the rat’s brain were re-running the maze from memory. This kind of “replay” is now considered strong evidence that sleep, and likely dreaming, plays a direct role in strengthening and organizing memories from the day.

Dogs and cats show the same REM twitching, whisker movement, and even vocalizations, and researchers who study animal sleep believe the underlying purpose is probably similar: consolidating memories and, quite possibly, mentally rehearsing everyday behaviors like chasing, hunting, or navigating familiar spaces, much like reindeer and other animals covered in our piece on how differently animals can approach the basic act of resting.

Why Did Dreaming Evolve?

So why would the ability to dream persist and spread across such a wide range of species? A few leading theories have emerged from this research:

• Memory consolidation: Dreaming and REM sleep appear to help strengthen and organize the day’s experiences into longer-term memory, as shown directly in Wilson’s rat replay studies.
• Skill rehearsal: Evidence from pigeons rehearsing songs and rats replaying spatial routes suggests some animals may use dream states to practice or reinforce important survival skills without physically moving.
• Problem-solving and recombination: Some researchers propose that dreaming allows the brain to combine unrelated pieces of information in new ways, potentially aiding creative problem-solving, a process conscious, logical thought doesn’t handle as freely.
• Threat simulation: A separate but related theory suggests dreaming may let animals mentally “rehearse” responses to danger, which could explain why birds react physically to recorded distress calls even while asleep.

Frequently Asked Questions

Do all animals dream?
Not necessarily, but REM-like sleep states have now been confirmed in a surprisingly wide range of species, including mammals, birds, reptiles, cephalopods like octopuses and cuttlefish, and jumping spiders. Whether every twitching animal is having a subjective dream experience the way humans do is still an open scientific question.

Do jumping spiders really dream?
Researchers have documented REM-like retinal movement and limb twitching in jumping spiders, which strongly suggests an REM-like sleep state. Scientists are cautious about claiming this proves actual dreaming, since confirming brain activity in a brain the size of a poppy seed is extremely difficult, but the behavioral evidence is compelling.

What do dogs dream about?
No one can say for certain, but based on related research in rats showing memory replay during sleep, most sleep researchers believe dogs are likely dreaming about everyday experiences, like running, chasing, or other familiar activities from their waking life.

Why do octopuses change color while sleeping?
During their brief “active sleep” phase, octopuses flash through colors and textures similar to the displays they use while awake and interacting with their environment, which researchers believe may be linked to a REM-like brain state, though whether this reflects an actual dream remains unconfirmed.

Final Thoughts

As technology enables researchers to study sleep states in new ways across increasingly diverse species, it’s becoming clear that dreaming, or at least dream-like brain and body states, confers real benefits that have driven its persistence throughout the animal kingdom. Gaining insight into this shared heritage, stretching from spiders to songbirds to octopuses, doesn’t just satisfy curiosity about our pets. It deepens our appreciation for just how widespread, and how ancient, the mysterious phenomenon of dreaming may really be.