As It Happens

The stuff of nightmares? Scientists think spiders may experience REM-like sleep

Scientists generally believe that all mammals sleep. But what about snoozing spiders? Behavioural ecologist Daniela Roessler spoke to As It Happens guest host Susan Bonner about her discovery that some might go even deeper.

Twitching limbs, uncontrollable movements point to REM-like-stage sleep, says researcher Daniela Roessler

A colour photo of a spider on a green piece of vegetation.
Researchers believe all animals sleep, and while sleep studies have not yet been conducted on spiders, a new study suggests a species of jumping spider might enter a deep-sleep-like stage of sleep, too. (Daniela Roessler)

Story Transcript

We already know that spiders have an uncanny ability to terrify people. But now researchers have observed the creatures entering a REM-like state — prompting the question: do spiders dream?

In a study published recently by a team of scientists from Germany, Italy and the U.S., researchers reported jumping spiders entering into a REM sleep-like state based on evidence of eye movement and twitching of their limbs. 

Daniela Roessler, a behavioural ecologist and postdoctoral researcher at the University of Konstanz, Germany was the first to notice the unusual behaviour in some eight-legged crawlers. She had collected the jumping spiders — a friendly species she affectionately calls a great "entry spider" due to their "cute" eyes and faces — for a different study altogether. But due to a COVID lockdown, she wound up bringing them home.

One night, she discovered them hanging from the lid of the container she had put them in. 

"That was very peculiar because I did not recall any such behaviour ever being reported or documented," Roessler told As It Happens guest host Susan Bonner.

A night-vision photo of a spider hanging upside down.
Daniela Roessler observed the spiders twitching and making uncontrolled movements. (Daniela Roessler)

Curiosity drove her to buy "a pretty cheap night-vision camera" that she souped up by attaching a couple of magnifying lenses with duct tape.

What she observed might be described by arachnophobes as the stuff of nightmares. 

"I saw this adult spider … just suddenly twitch, and she kind of curled up all the legs and she was making these really uncontrolled moves." 

Roessler wondered if the creature was sick — or worse, dying. But further research revealed that the movement happened at regular intervals — which is when she says they started hypothesizing that they were actually experiencing something like REM — or rapid eye movement — sleep.

Though she said classic sleep studies have not yet been done on spiders, Roessler said researchers around the world agree that all animals sleep. 

"So we're quite confident that they are asleep," she said of the spiders. "But we will still have to show it."

A black and white photo of a scientist with shoulder-length hair.
Daniela Roessler, a behavioural ecologist from the University of Konstanz, Germany, made the accidental discovery about the spider sleep. (Ines Janas)

If they can do that, she says, adding the notion of REM sleep will be even more important. 

"This will add a pretty big piece to the puzzle of how sleep evolved and where it originated," she said, "because so far, REM sleep is something that is pretty special to mammals and birds and kind of connected to big, complex brains." 

Indeed, many people have seen what they believe is their dog or cat dreaming — moving their legs, making noises, scrunching up their noses — all while fast asleep. 

But Roessler does point out that while we might all think it's dreaming, it will probably remain impossible to prove.

"We can't really look into their brain," she said. But she says considering they are visual creatures with so much sensory input during the day, it would make sense that they are dreaming. 

A close up of a dark spider hanging upside down.
The finding could help scientists to test fundamentals of REM sleep in creatures with less complex brains than humans. (Daniela Roessler)

"Whether that is the same thing or even anything close to what we experience as dreams is of course a completely different question." 

Oh, and just what might a jumping spider dream of? Roessler says that given that dreaming is likely connected to memory formation and learning and experiences, a spider's dreams would likely reflect that too. 

"So maybe they're dreaming about a mate, or how to approach the mate or about a fly they just ate. Or maybe how to make a really perfect, perfectly targeted jump."

Written by Stephanie Hogan. Interview produced by Katie Geleff. 

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