Even Spiders Catch Up On Sleep After A Rough Night

Spiders Have a Twitchy Sleep Stage Resembling REM in Mammals

Lose a night of sleep, and the body tends to make up for it later. A small jumping spider from Queensland, Australia, appears to do the same. After one night pestered by fruit flies, the spiders returned the next night with about 65% more bursts of twitching, according to a new study in the Proceedings of the National Academy of Sciences. The bursts, which involve twitching legs and shifting eyes, resemble REM sleep in mammals. REM, short for rapid eye movement, is the sleep stage marked by darting eyes and twitching limbs.

An earlier report of similar twitching in a different jumping spider raised an obvious question: was the animal really asleep, or just resting? Scientists use a behavioral checklist to tell the difference, including a distinct sleeping posture, long stretches of stillness, and being harder to rouse. The species studied here, Portia fimbriata, passed all of it.

That matters because sleep science leans heavily on mammals and birds, whose brains are built very differently from a spider’s. Octopuses and fruit flies have recently shown signs of sleeping in stages too, and spiders now have behavioral evidence pointing the same way.

Spider Sleep Has a REM-Like Stage Marked by Twitching

Researchers studied 92 spiders, all lab-raised offspring of spiders from a captive breeding program in Queensland, kept on a 12-hour light and 12-hour dark schedule. Portia is an unusually clever hunter that preys on other spiders.

First came a test of whether the spiders have an internal body clock. Forty were watched by infrared camera for 60 hours under normal day-night lighting, constant dim light, or constant bright light. Their cycle of rest and activity carried on in constant dim light, and weakened but did not vanish in constant bright light, a sign that the rhythm comes from inside the spider rather than from changes in light.

Then 10 spiders were filmed overnight. At dusk, the spiders searched for a sleeping spot, often a silk retreat, and hung upside down beneath a silk platform with their rear ends sagging, a posture unlike their daytime stance. Sleep began about 15 minutes after the lights went out and lasted nearly 11 hours. Twitching came in bursts, averaging about 11 a night among those 10 spiders, each lasting around 80 seconds. The first showed up about four hours in on average, though timing varied widely from spider to spider.

Four of 10 Spiders in REM-Like Sleep Ignored a Sound

Twitching alone does not prove a different brain state, since it could be random muscle jitter. So 30 adult spiders, half male and half female, were each tested once with a tone that slowly grew louder over 10 seconds. Ten spiders were awake, 10 had been motionless for at least 15 minutes, and 10 were mid-twitch.

Awake spiders reacted in under two seconds on average. Motionless ones took a bit longer, and twitching spiders were slowest, at more than four seconds. Every awake and motionless spider reacted within the 10 seconds, but only six of the 10 twitching spiders did. The other four responded right away when retested awake, which rules out a hearing or movement problem.

One catch: twitching spiders were reliably slower than awake ones, but the gap between twitching and plain motionless spiders was too small to rule out chance. Motionless spiders also varied widely, with some reacting as fast as awake ones and others as slowly as twitching ones. That hints that stillness includes both light rest and deeper sleep, though the sample was small and the authors call the idea tentative.

Disrupted Spider Sleep Brought More REM-Like Episodes the Next Night

A final experiment asked whether spiders need this sleep. Seventeen spiders each spent one night with 25 live fruit flies in their enclosure. Flies suit the job because Portia specializes in hunting other spiders, so they jostle it without triggering a full hunt. Spiders stayed in resting postures but were repeatedly startled, shifted around, and sometimes tried to dislodge the flies. Usable recordings came from 13 of them.

On the next night, twitching episodes became more frequent without getting any longer, and overall sleep time grew by about 8.5%. A control group of five undisturbed spiders, tracked for three nights, did not sleep longer as the nights went on, so the extra rest traced back to the flies.

Why a spider would need a twitchy stage is an open question. The authors note that Portia lives in a dense rainforest, where it must make sense of constant motion, light, and shape, and that REM sleep in mammals has been linked to learning and memory. A spider’s brain may use the bursts to sort through the day’s sights, though that idea was not tested. Brain waves were not measured either, so some quiet stretches may have been calm wakefulness.

Spiders appear to need their sleep, twitching included. Behavior alone makes a strong case that they sleep in stages, and brain recordings will show how close the match to mammal REM really is.

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