Small jumping spiders dangle from their threads at night inside a box in a lab. Every now and then, their legs curl and their spindles tremble. The retinas of their eyes, visible through their transparent exoskeletons, move back and forth.

Dr. Daniela Rösler, a behavioral ecologist at the University of Konstanz in Germany, says, "What happens to those spiders seems similar - to a large extent - to what is known as REM sleep." During that sleep stage, the eyes of the sleeping animal move in unpredictable ways.

For humans, REM sleep is the stage in which they experience dreams, particularly dreams rich in realistic details. This leads us to a puzzling question: If spiders experience REM-like sleep, does that mean there are dreams unfolding in their brains, which are the size of a poppy seed?

Rösler and her colleagues conducted a study on the orb-weaving spiders in 2022, where they placed monitoring cameras above 34 spiders and found that they experience sleep stages resembling REM sleep approximately every 17 minutes. The eye movement behavior was limited to those episodes, as it did not occur during the nighttime periods when the jumping spiders were moving, stretching, adjusting their silk threads, or cleaning themselves with one of their legs.

Although the spiders seem inactive in the lead-up to the REM-like episodes, the team has not yet been able to prove that they are asleep. However, if it turns out that this happens during their sleep, Rösler says, and if what appears to be REM-like movement is indeed so, dreams would be a distinct possibility. She adds that it is easy for her to imagine that jumping spiders, which rely heavily on their vision, might benefit from dreams as a way to process the information they acquire during the day.

Rösler is not the only researcher trying to answer those questions regarding animals far removed from humans in the evolutionary chain. Other scientists have discovered evidence that a broader range of animals experience REM sleep stages: spiders, lizards, and zebrafish. This has led researchers to wonder if dreams, once thought to be exclusive to humans, are more common than they had assumed.

REM sleep is characterized by several features beyond rapid eye movements: temporary paralysis of skeletal muscles, repetitive twitches of body parts, and increased brain activity along with elevated breathing and heart rates. This type of sleep was first observed in newborn infants in 1953, and soon after, it was detected in other animals such as cats, rats, horses, sheep, opossums, and armadillos.

Brain activity during this sleep stage has distinctive features, at least in humans. During the stages without rapid eye movements, known as quiet sleep phases, brain activities are synchronized. Neurons fire their signals simultaneously and then pause, particularly in the cerebral cortex, creating what is known as slow wave activity. In contrast, during the REM sleep phase, bursts of electrical activity emerge in the brain similar to what occurs during wakefulness.
Even among mammals, REM sleep stages are not uniform. Naked mole-rats, mammals from the group of marsupial animals, exhibit some features of both REM sleep and quiet sleep simultaneously. Studies conducted on whales and dolphins suggest that they may not experience REM sleep at all, while birds do experience these phases, which are characterized by beak twitching and relaxation of the muscle that enables them to keep their heads raised.

Researchers have begun to discover sleep-like states in many species and animal types.

In 2012, for example, researchers observed a sleep-like state in cuttlefish, with behaviors resembling REM sleep: periodically, the eyes of these animals moved rapidly, their arms trembled, and the colors of their bodies changed. During a research grant at the Woods Hole Marine Biological Laboratory in Massachusetts, behavioral biologist Teresa Iglesias studied this phenomenon and recorded six cuttlefish over long hours.

All six fish exhibited activity resembling that which happens during REM sleep approximately every 30 minutes: episodes of arm and eye movements during which various patterns and colors appeared on their skin. These creatures also emitted camouflage and attention-grabbing signals, both of which are part of their wakeful behavior. Since the brain of these cephalopods directly controls the change in skin patterns and colors, this "indicates that brain activity is somewhat out of control" during sleep, says Iglesias, who now works at the Okinawa Institute of Science and Technology in Japan.

Researchers later observed a similar state in octopuses. Iglesias says that if octopuses and cuttlefish dream, "that somewhat undermines our belief that humans are extremely distinctive beings."

Researchers have also recorded a REM-like stage in bearded dragons by measuring the signals from electrodes in their brains. They also indicated the presence of at least two sleep states in zebrafish based on their brainwave patterns. In one of these stages, brain activity occurs synchronously, similar to what happens in mammals during quiet sleep. In another stage, the fish exhibited nervous activity akin to that which occurs during wakefulness and REM sleep (but the fish did not display rapid eye movements).

By observing multiple sleep stages in species that are evolutionarily distant from humans, researchers indicated that different types of sleep began hundreds of millions of years ago. It is now known that flies can also switch between two or more sleep stages. As for nematodes, they appear to undergo only one sleep stage.

Researchers are studying the likelihood of animals dreaming during REM sleep since creatures exhibit behaviors similar to their awake states during that phase – such as the color pattern changes of cuttlefish or the tremors of spider spindles. Janina Ongurian, a sleep scientist at the Max Planck Institute for Biological Intelligence in Munich and at Göttingen Medical School, and her colleagues observed that the pupils of pigeons constrict during REM sleep, just as they do during the courtship behavior of those birds. Ongurian states that this raises the question of whether pigeons dream, or somehow re-experience what they went through during wakefulness.

There is also a connection between REM sleep and the replaying of scenarios in certain animals. For example, when researchers examined the electrical activity in the brains of sleeping mice that had run through a maze during wakefulness, they noted that neurons released signals to help them identify places. They also recorded activity in neurons associated with eye movement. These two factors suggest that mice may be going through an experience akin to dreaming in which they scan their surrounding environment, according to Ongurian.

She adds that if those clues are taken into account, we can hypothesize that animals may be dreaming. "But when we look at those reasons individually, it becomes clear that none of them is sufficient." The brain activity associated with replaying experiences in mice does not only happen during REM sleep, she states. It also occurs during planning or daydreaming. The connection between REM sleep and dreaming is not definitively fixed: humans dream during quiet sleep as well, and when drugs were used to inhibit REM sleep in one study, that did not prevent participants from experiencing long and bizarre dreams.

Ongurian emphasizes that ultimately, people know they are dreaming because they can talk about their dreams, "but animals cannot do that, and this is the biggest problem when we try to verify this scientifically accurately."

There remains debate over the benefits of REM sleep. Professor Paul Shaw, a neuroscientist at Washington University in St. Louis, states, "No one knows definitively what the function of sleep is – whether in REM sleep or quiet sleep." Among the more accepted ideas is that REM sleep helps the brain to form and reorganize memories, while other theories suggest that this sleep stage aids in brain development and bodily movement systems, and also maintains the electrical circuits necessary for wakeful activities from deteriorating during sleep.