Spiders snooze just like humans – and may even dream, study says

Spiders twitch during sleep and may even have dreams just like humans do, a new study shows. 

Researchers in Germany filmed more than 30 baby jumping spiders as they slept using an infrared camera. 

As they rested, they displayed ‘periodic bouts of retinal movements’ coupled with ‘limb twitching and leg curling’, the experts found. 

This is akin to REM sleep in humans, when our eyes move rapidly from side to side behind closed eyelids as we experience vivid dreams. 

So the results suggest spiders also experience REM sleep, and therefore may have dreams too. 

The study was led by Daniela Roessler, an evolutionary biologist at the University of Konstanz in Germany, and published in the journal Proceedings of the National Academy of Sciences. 

‘Here, we report evidence for an REM sleep-like state in a terrestrial invertebrate – periodic bouts of retinal movements coupled with limb twitching and stereotyped leg curling behaviors during nocturnal resting in a jumping spider,’ they say.

‘Observed retinal movement bouts were consistent, including regular durations and intervals, with both increasing over the course of the night. 

‘This report provides direct evidence for an REM sleep-like state in a terrestrial invertebrate – an arthropod – with clear parallels to REM sleep in terrestrial vertebrates.’ 

Other animals, including some birds and mammals, have been shown to experience REM sleep.

But spiders haven’t had as much attention in this area, so it wasn’t known if they got the same kind of sleep, said Roessler. 

The researchers studied Evarcha arcuata, a species of jumping spider that suspends itself upside down on a silk line to rest. 

Being nocturnal, the species was filmed between 7pm and 7am, whilst hanging threads of silk in their lab containers. 

In humans, sleep is generally separated into ‘non rapid eye movement’ or NREM sleep and rapid eye movement or REM sleep. A typical night’s sleep goes back and forth between the stages

Other animals, including some birds and mammals, have been shown to experience REM sleep. But creatures like the jumping spider haven’t had as much attention so it wasn’t known if they got the same kind of sleep, said Roessler

Many species similar to spiders actually don’t have movable eyes, which makes it hard to compare their sleep cycles.

But these jumping spiders are predators that move their retinas around to change their gaze while they hunt. 

Plus, the young spiders have a see-through outer layer that gives a clear window into their bodies. As an adult, it looks very different, with a furry brown body and four pairs of big eyes.

The research showed the spiders’ sleep movements looked a lot like REM in other species, too – not just humans, but dogs or cats twitching in their sleep. 

Insects like the jumping spider are obviously far from humans on the evolutionary tree, so the link with humans regarding sleep behaviour is a curious one.

As an adult, Evarcha arcuata, a species of jumping spider, has a furry brown body and four pairs of big eyes

The researchers still have to figure out if the spiders are technically sleeping while they’re in these resting states, Roessler said. 

That includes testing whether they respond more slowly, or not at all, to triggers that would normally wake them up.

Barrett Klein, an entomologist at the University of Wisconsin-La Crosse who was not involved with the study, said it was exciting to find REM-like signs in such a distant relative. 

But questions remain about how widespread REM sleep is and what purpose it might serve for species, he added.

REM sleep is ‘still very much a black box,’ Klein said.

Meanwhile, Jerry Siegel, a sleep researcher who was also not involved with the study, said he’s doubtful that the spiders can really experience REM sleep.

‘There may be animals that have activity in quiet states,’ said Siegel, of the UCLA Center for Sleep Research.

‘But are they REM sleep? It’s hard to imagine that they could be the same thing.’