Do spiders sleep? Study suggests arachnids snooze just like humans – and may even DREAM
- Experts filmed more than 30 baby jumping spiders as they slept during the night
- They showed ’bouts of retinal movements’, much like humans during REM sleep
- During REM sleep, our eyes move rapidly from side to side behind closed eyelids
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.
Researchers in Germany filmed more than 30 baby jumping spiders as they slept using an infrared camera
WHAT IS REM SLEEP?
REM is a kind of sleep that occurs at intervals during the night and is characterized by rapid eye movements.
REM sleep first occurs about 90 minutes after falling asleep – and tends to be when we experience our most vivid dreams.
During REM sleep, our eyes move rapidly from side to side behind closed eyelids.
Most dreaming occurs during REM sleep, although some can also occur in non-REM sleep.
Arm and leg muscles become temporarily paralysed, which prevents us from acting out our dreams.
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.
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.’
THE FOUR STAGES OF SLEEP
Pictured, different steps of the night sleep cycle. Most dreaming occurs during REM sleep (marked in red) although some can also occur in non-REM sleep
Sleep is generally separated in four stages. The first three of these are known as ‘non rapid eye movement’ or NREM sleep.
The last stage is known as rapid eye movement or REM sleep.
A typical night’s sleep goes back and forth between the stages.
Stage 1: In the first five minutes or so after dropping off we are not deeply asleep.
We are still aware of our surroundings but our muscles start to relax, the heart beat slows down and brainwave patterns, known as theta waves, become irregular but rapid.
Although we are asleep during Stage 1, we may wake up from it feeling like we didn’t sleep at all.
After around five minutes our bodies move into stage two.
Stage 2: This is when we have drifted into sleep, and if awakened would know you we been asleep. Waking up is still fairly easy.
This stage is identified by short bursts of electrical activity in the brain known as spindles, and larger waves known as K-complexes, which indicate that the brain is still aware of what is going on around it before turning off to a sub-conscious level.
Heartbeat and breathing is slow, and muscles relax even further.
Our body temperature drops and eye movements stop.
Brain wave activity slows but is marked by brief bursts of electrical activity.
Stage 3: Stage 3 non-REM sleep is the period of deep sleep that we need to feel refreshed in the morning.
It occurs in longer periods during the first half of the night.
Our heartbeat and breathing slow to their lowest levels during sleep and brain waves become even slower.
Our muscles are relaxed and it people may find it difficult to awaken us.
The body repairs muscles and tissues, stimulates growth and development, boosts immune function, and builds up energy for the next day.
Hypnagogia – the transitional state between wakefulness and sleep – is associated with NREM stages one to three.
Mental phenomena during hypnagogia include lucid thought, lucid dreaming, hallucinations and sleep paralysis.
REM sleep: REM sleep first occurs about 90 minutes after falling asleep.
Our eyes move rapidly from side to side behind closed eyelids.
Mixed frequency brain wave activity becomes closer to that seen in wakefulness.
Our breathing becomes faster and irregular, and heart rate and blood pressure increase to near waking levels.
Most dreaming occurs during REM sleep, although some can also occur in non-REM sleep.
Arm and leg muscles become temporarily paralysed, which prevents us from acting out our dreams.
As we age, we spend less of our time in REM sleep.
Memory consolidation most likely requires both non-REM and REM sleep.
Source: US National Institutes of Health
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