Are spiders dreaming? Spiders may have a REM sleep state

Like the active sleep of dreamy people, jumping spiders (Evarcha arcuate) may also involve a state of rapid eye movement (REM) sleep – according to new research published in PNAS.

An international team of researchers studied the retinal movements of jumping spiders during sleep and found that they coincided with body movements associated with REM sleep in other animals.

“This report provides direct evidence for a REM sleep-like state in a terrestrial invertebrate – an arthropod – that has clear similarities to REM sleep in terrestrial vertebrates,” say the authors. “The combination of periodic limb twitches and eye movements during this sleep-like state, as well as the increased cycle duration similar to REM sleep, fulfills the basic behavioral criteria for REM sleep observed in vertebrates, including humans .

“Eye movement patterns during REM sleep have been hypothesized to be directly linked to the vision seen while dreaming—begging the deeper question of whether jumping spiders have visual dreams,” they said.

And if spiders dream – as a certain iconic science fiction novel from 1968 asks – do invertebrate sheep dream?

Read more: REM sleep is the key to dream imagery

In many animals sleep occurs in alternating periods of two distinct states – one when there is no movement called non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep.

As you might expect, during the REM stages of sleep, rapid, jerky eye movements occur. But it is also characterized by sleep paralysis that suppresses most body movements but allows muscle twitches on a smaller scale in the limbs, as well as brain waves similar to the activity when they wake up.

Importantly, it is during REM sleep that we dream.

Although the best indicator of REM sleep is eye movements, only a limited number of lineages across the animal kingdom have come up with movable eyes. This makes it difficult to compare REM sleep across species, and scientists still do not know its evolutionary origins and function.

​​​​The team previously discovered that jumping spiders suspend themselves upside down and remain inactive on a silk line throughout the night, suggesting that they may be sleeping. During this period, the spiders show activity phases including leg curling and twitching, which the researchers hypothesized could be indicative of a REM sleep state.

But while jumping spiders can’t move their eye lenses, they can move their retinal tubes to adjust their vision. And since the exoskeletons of baby spiders lack pigment, researchers could use infrared (IR) cameras to look directly at them and observe them during the night.

The authors recorded and analyzed nighttime IR footage of 34 young jumping spiders and found that the spiders exhibited consistent periods of retinal movements at regular intervals. Not only that, but those periods and their duration increased during the night.

Importantly, they also coincided with leg curling (broad contractions of the leg towards the sternum) and limb flapping – similar to the movements seen in other animals during REM sleep.

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