The Thing of Your Nightmares Might Actually Have Dreams

You've probably seen a dog kick its legs mid-dream, as though running. Spiders apparently do much the same thing, according to a new study—the first to indicate that arachnids dream. Daniela Roessler, a postdoctoral fellow at Germany's University of Konstanz, trained infrared cameras on sleeping spiders at night and recorded twitches in the body and limbs that she believes are evidence of rapid eye movement sleep, or REM sleep—the stage of sleep associated with dreaming. The baby jumping spiders—which have translucent heads, allowing excellent viewing of their lidless eyeballs—twitched their bodies and curled their legs, while their retinal tubules "shifted rapidly to indicate that they were experiencing rapid-eye-movement," per the Wall Street Journal.

"When I saw for the first time these twitching faces, it just blew my mind because it looked like when cats or dogs dream," says Roessler, lead author of the study published Monday in Proceedings of the National Academy of Sciences. This has been seen in animals with backbones but "never before observed in arachnids," per National Geographic. Paul Shaw, a professor of neuroscience at Washington University School of Medicine, who wasn't involved in the research, said the finding "expands the number and types of animals that exhibit this human state" and "changes the landscape of when REM evolved," per the Journal. Just as with humans, the spiderlings appeared to cycle through periods of REM. They occurred every 20 minutes and lasted 77 seconds, per NatGeo.

"I do think that they are dreaming. But scientifically proving that is going to be a different story," Roessler told the Journal. She noted it’s difficult to measure activity in the spiders' brains, which are the size of poppyseeds. But future research will compare spiders' eye moments during REM sleep and during wakefulness. If the same pattern observed when a spider catches a fly is observed during REM sleep, that would suggest that the spider is dreaming of that activity, Roessler said. She told NatGeo that the spiderlings' silk-producing spinneret organs would periodically "go nuts" during observation, indicating that they are "practicing" a waking behavior. After all, jumping spiders "constantly set little silk anchors wherever they go." (More spiders stories.)