Scientists have discovered that spiders experience REM-like sleep, complete with twitching muscles and moving eyes, raising the intriguing possibility that they might dream.
Spiders may have more in common with sleeping humans than scientists previously realized. Researchers at the University of Bonn have discovered that Australian jumping spiders experience two distinct stages of sleep, including a state that closely resembles REM sleep in humans. During this stage, the spiders move their eyes and twitch their muscles, behaviors that raise intriguing questions about the origins of sleep and whether spiders might experience something resembling dreams.
The findings provide the first comprehensive experimental confirmation of regulated sleep in spiders. They also suggest that complex sleep patterns may be far more widespread throughout the animal kingdom than previously understood.
In humans and other vertebrates, sleep involves multiple stages, including REM (rapid eye movement) sleep. This stage is associated with rapid eye movements, changes in muscle activity, and vivid dreaming in humans. Scientists have long wondered whether animals with very different nervous systems, including spiders, experience comparable sleep states.
To investigate, researchers from the Institute of Organismic Biology at the University of Bonn turned to an unusual subject: the Australian jumping spider Portia fimbriata.
A Remarkably Intelligent Spider
The spider belongs to the genus Portia, a group of predators known for their sophisticated hunting techniques. Unlike many spiders that primarily capture insects, Portia species specialize in hunting other spiders.
One of their most remarkable strategies involves manipulating the webs of their prey. By producing vibrations that imitate insects struggling in silk, these spiders can lure web-building spiders toward them. Such complex hunting behavior suggests an unusual capacity for learning and adapting to different situations.
Despite their impressive abilities, these predators are quite small. A fully grown Portia fimbriata is roughly the size of a one-euro coin.
Their sophisticated behavior made them particularly interesting subjects for investigating sleep. Scientists wanted to determine whether these spiders simply become inactive at night or enter a genuine, biologically regulated sleep state similar to those observed in vertebrates.
Scientists Discover Two Distinct Sleep Stages
The researchers found convincing behavioral evidence that Portia fimbriata does more than rest. The spiders exhibit regulated sleep with two clearly distinguishable stages.
One of these stages closely resembles REM sleep, a well-known phase of human sleep characterized by rapid eye movements and muscle twitches.
During the spiders’ REM-like periods, the researchers observed eye movements and twitching muscles, suggesting that certain features of sleep may be shared by animals whose nervous systems are dramatically different from our own.
The discovery is especially significant because most scientific knowledge about sleep has come from studies of vertebrates, including humans, other mammals, birds, and reptiles. Until now, comprehensive experimental evidence establishing true sleep behavior in spiders had been lacking.
Although the REM-like movements are intriguing, they do not demonstrate that spiders dream. Determining whether spiders experience anything comparable to human dreams would require evidence beyond the behavioral observations available in this study.
How Researchers Studied Sleeping Spiders
To examine the spiders’ nighttime behavior, the scientists monitored individual Portia fimbriata specimens over multiple days and nights.
Each spider was kept in a small plastic dish measuring 12 centimeters in diameter. The researchers exposed the animals to different lighting conditions to investigate their daily activity cycles and determine how their behavior changed between day and night.
High-resolution video recordings allowed the team to examine the spiders’ movements in detail. During nighttime observations, infrared cameras captured their sleeping positions, periods of inactivity, and episodes of REM-like behavior without requiring visible light.
But remaining motionless is not necessarily proof of sleep. Animals can become inactive for several reasons, so the researchers needed additional evidence to distinguish actual sleep from ordinary rest.
They tested the spiders’ responses to sound and measured how quickly the animals reacted. These experiments helped determine whether the spiders became less responsive to external disturbances during their resting periods, an important characteristic of sleep.
The team also investigated a phenomenon known as “sleep rebound,” which occurs when an animal compensates for interrupted or insufficient sleep by sleeping more afterward. Testing for this response helped establish whether the spiders’ rest was regulated by an underlying biological need.
Together, these observations and experiments provided evidence that the spiders experience genuine sleep rather than simply remaining inactive.
The Challenge of Identifying Sleep in Spiders
Studying sleep in spiders presents difficulties that researchers do not typically encounter when investigating humans or other mammals.
One major obstacle was identifying a stimulus that would reliably awaken the spiders during experiments. Spiders perceive their surroundings very differently from humans, making it challenging to design tests that could consistently measure their responses.
The researchers also had to be careful when interpreting the unusual movements recorded during the REM-like stage. Although eye movements and muscle twitches resemble behaviors associated with REM sleep in vertebrates, similar outward movements do not necessarily indicate identical processes inside the nervous system.
Another important limitation was the inability to directly measure the spiders’ brain activity. Instead, the researchers had to base their conclusions entirely on observable behavior.
To overcome these challenges, they collected extensive high-resolution video recordings over long observation periods. This provided the detail needed to examine subtle movements and sleep patterns, but it also generated enormous amounts of data that had to be analyzed.
By combining behavioral recordings with experiments measuring responsiveness and recovery from interrupted sleep, the researchers were able to build a much stronger case for true sleep in spiders.
What Spider Sleep Reveals About Evolution
The discovery could have important implications for understanding how sleep evolved.
REM sleep is one of the most fascinating and still incompletely understood aspects of human biology. Although it is closely associated with dreaming in humans, scientists continue to investigate its broader functions and evolutionary history.
Finding REM-like behavior in spiders raises the possibility that some features of complex sleep are shared across a much wider range of animals than previously established.
Spiders and vertebrates have very different nervous systems, yet both appear capable of entering distinct behavioral sleep states. Understanding these similarities could help researchers identify fundamental properties of sleep that have persisted through evolution or developed independently in different animal groups.
The findings also open new opportunities in behavioral biology, neuroscience, and chronobiology, the study of biological rhythms and internal clocks. They may be particularly valuable for understanding how sleep functions in animals with relatively simple nervous systems.
Rather than limiting sleep research to humans and traditional laboratory animals, the scientists recommend studying a much broader diversity of species, especially those with unusual nervous systems or highly specialized adaptations.
Such comparisons could help reveal whether sleep serves a universal biological purpose and whether some of its essential features originated very early in animal evolution.
Despite decades of research, many basic questions about why animals sleep and what happens during different sleep stages remain unanswered, even in humans.
Could Spiders Dream? Scientists Want to Find Out
The University of Bonn researchers are now extending their investigations to other spider species to better understand how sleep and REM-like states evolved across this diverse group of animals.
They also plan to compare spiders sleeping in laboratory environments with those living in the wild. These studies could reveal whether natural surroundings influence sleep patterns in ways that controlled laboratory conditions cannot fully capture.
Another major research direction involves exploring the relationship between sleep and intelligence. Because Portia spiders display sophisticated hunting and learning abilities, they provide an interesting opportunity to investigate whether sleep contributes to cognitive processes in animals with very different brains from humans.
Future studies will also examine what happens inside the nervous systems of spiders and insects during sleep. Direct measurements of neural activity could help determine whether the REM-like behaviors observed in spiders involve processes comparable to those found in vertebrates.
The researchers are additionally interested in how environmental changes influence sleep. In particular, they hope to investigate the effects of climate change and human activities, including artificial light at night (ALAN), on the sleep behavior of spiders and other small animals.
Artificial nighttime lighting can alter the natural cycle of light and darkness that many organisms use to regulate their daily rhythms. Understanding how spiders respond to these disruptions could provide new insights into the broader ecological consequences of human activity.
For now, the findings establish something remarkable: even a tiny jumping spider can experience distinct, regulated stages of sleep, including one that resembles REM sleep in humans.
Whether those sleeping spiders actually dream remains an open question, but their unusual nighttime behavior offers scientists a new way to investigate one of biology’s oldest mysteries.
Reference: “Behavioral evidence for two-stage sleep in spiders” by Nadja Geiger, Maxim Q. Capelle, Phillip Rosentreter, Marie E. Herberstein, Armin Bahl, Massimo De Agrò, Barrett A. Klein and Daniela C. Rößler, 26 October 2026, Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2618030123
This work was supported by the Emmy Noether Program of the German Research Foundation (DFG).
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