A spider most people would walk past without a second look just became one of the more interesting data points in sleep science. Researchers at the University of Bonn, publishing in the Proceedings of the National Academy of Sciences, report that the jumping spider Portia fimbriata cycles through two distinct, behaviorally defined sleep stages overnight — including a REM-like stage that responds to sleep loss the same way mammalian REM sleep does.
Testing Whether It's Really Sleep
Confirming that an animal sleeps, rather than simply resting, requires more than watching it go still. The Bonn team, led by Daniela Rößler with Nadja Geiger as a key collaborator, applied the standard behavioral criteria used across the animal kingdom: a stereotyped resting posture, an elevated threshold for waking the animal up, organization around a circadian rhythm, and — the most demanding test — a homeostatic rebound after sleep is disrupted.
Portia fimbriata passed all four. The spiders settled into a consistent quiescent posture at night, became harder to arouse than during active periods, and kept their rest cycles tied to a day-night rhythm. The team identified a quiet, low-activity stage early in the rest period followed by a second stage marked by structured micromovements — leg curls, abdominal twitches, and retinal movements — that closely resembles the behavioral signature of REM sleep in vertebrates.
The Rebound Effect
The clearest evidence came from a sleep-deprivation experiment. When researchers kept spiders awake overnight and then let them rest normally, the animals spent roughly 80% more time in the REM-like stage the following night compared with undisturbed controls. That kind of compensatory rebound — the body appearing to "catch up" on a specific stage of sleep it was denied — is one of the strongest behavioral markers scientists use to distinguish true sleep from simple inactivity, and it had not previously been demonstrated this clearly outside vertebrates in a jumping spider.
The finding builds on a 2022 PNAS paper from a different research group, which recorded twitching and retinal movements in a related jumping spider species, Evarcha arcuata, but could not establish that the animals were actually asleep rather than merely resting with reduced muscle tone. The new work closes that gap for Portia fimbriata by pairing the behavioral observations with the homeostatic rebound test.
What the Study Can't Yet Say
The researchers are careful about the limits of their findings. No one recorded neural activity in these spiders, so there is no way to confirm whether the REM-like stage involves brain activity patterns resembling those seen in sleeping mammals and birds — only that the behavioral markers line up. Jumping spiders have a relatively sophisticated visual system and complex, vertebrate-like hunting behavior for a tiny predator, which has made them a longstanding subject of interest for researchers probing where cognition-related biology begins on the evolutionary tree. Whether anything resembling dreaming occurs remains, as the researchers acknowledge, entirely unanswerable with current methods. The work also covers a single species, so broader claims about spiders or arachnids generally will need replication elsewhere.
What This Means for Patients
This is basic biology, not a clinical finding, and it changes nothing about how sleep disorders are diagnosed or treated in people today. Its value is in the bigger picture: sleep, and specifically a REM-like stage with homeostatic regulation, now appears in animals separated from humans by roughly 600 million years of evolution and a nervous system built on an entirely different architecture. That pattern strengthens the case that whatever sleep accomplishes — and REM sleep in particular — serves a function so fundamental that evolution has preserved some version of it again and again, independently, across radically different kinds of brains. For researchers trying to understand why humans suffer real health consequences when REM sleep is disrupted, by insomnia, sleep apnea, certain medications, or other causes, comparative evidence like this helps separate what is a quirk of mammalian biology from what is a deeper, non-negotiable requirement of having a nervous system at all.
