Spiders Need Rest, Too. But Do They Sleep?

By Gina Wynn

We’re familiar with cat naps, but what about spider naps? Researchers who conducted a study of baby jumping spiders at the University of Konstanz in Germany found that they may indeed experience sleep cycles. Results of the study were published in the article “Regularly occurring bouts of retinal movements suggest an REM sleep-like state in jumping spiders,” which appeared in the journal Proceedings of the National Academy of Sciences.

Daniela Roessler stumbled upon the idea of studying spider sleep patterns when she noticed spiders in the lab hanging from silk threads in their containers at night. While some aquatic animals, roundworms, and other arthropods have recently been shown to experience sleep cycles, it was unclear whether creatures like jumping spiders exhibited similar habits.

Critters on Camera

To gain insight into the hanging behavior, Roessler and her team observed 34 four-eyed, furry arachnids at night using infrared cameras. Over three four-hour phases, they saw parts of their eyes flickering and their legs twitching and curling. They noted that these behaviors resembled activity typical of rapid eye movement (REM) sleep-like states in other species­—like dogs or cats—who twitch during sleep, and that the movements occurred in regular cycles, as they do in humans.

Often associated with mammals and birds, REM sleep states alternate with non-REM sleep states several times throughout a normal night of sleep. They are characterized by increased activity in certain brain regions, depressed muscle tone, and rapid, multi-directional eye movements, and are associated with dreaming in humans.

Comparing sleep cycles in spiders to other species can be challenging, as many spider-like creatures lack movable eyes. However, jumping spiders, which are predatory and have movable retinas so they can look around while hunting, provided a unique opportunity for observation.

Reservations About REM

Because spiders are so different from humans evolutionarily, some experts have reservations about whether spiders truly experience REM sleep. Jerry Siegel from the UCLA Center for Sleep Research is skeptical, according to the AP News article “Do spiders sleep? Study suggests they may snooze like humans.” He believes that while some animals may display activity during rest, it's hard to consider it REM sleep.

Barrett Klein from the University of Wisconsin-La Crosse told AP News that the discovery of REM-like signs in spiders is exciting, but the prevalence of REM sleep among different species and its purpose remains unclear. Neither Siegal nor Klein were involved with the study.

Let Sleeping Spiders Lie

To determine the nature and significance of spider sleep cycles and their evolutionary implications, the researchers have more work to do. Roessler told AP News that they still need to understand whether the spiders are technically sleeping during their resting states. Further investigations will involve testing spiders while they appear to be sleeping to find out whether they respond more slowly or not at all to stimuli that would typically provoke a reaction.

As they delve deeper, the team hopes to answer important questions about the visual brain and the origin, evolution, and function of REM sleep. In short, they plan to let sleeping spiders lie...while keeping a watchful eye.