(CN) — The rapid color changes of a sleeping octopus indicate two major alternating sleep states, according to a new study published in the journal iScience.
Like humans, octopuses have two stages of sleep: active and quiet, researchers at the Brain Institute of the Federal University of Rio Grande do Norte, Brazil, discovered. It is during the active state that color changes pulse across their skin, signaling the possibility that the most intelligent of the cephalopods may even dream — a characteristic long thought limited to mammals, birds and select reptiles, including the bearded dragon.
“That led us to wonder whether we might see evidence of two sleep states in octopuses, too,” senior author Sidarta Ribeiro said. “Octopuses have the most centralized nervous system of any invertebrate and are known to have a high learning capacity.”
For years, scientists have known that octopuses have a complex nervous system and are among the most intelligent of all invertebrates. The discovery that these soft-bodied shape shifters have sleep cycles could expand human understanding of the evolution of sleep.
To reach this conclusion, researchers made video recordings of four Octopus insularis in a lab. The videos demonstrated that during “quiet sleep,” the octopuses were still and quiet, their skin pale and eye pupils contracted to a slit. However, during “active sleep,” the cephalopods dynamically changed their skin color and texture as they moved their eyes while contracting their suckers and body with muscular twitches.
“What makes it more interesting is that this ‘active sleep’ mostly occurs after a long ‘quiet sleep’ —generally longer than 6 minutes,” Ribeiro said.
The cycle would repeat every 30 minutes or so.
To ensure the octopuses were sleeping, researchers used visual and tactile stimulation tests to measure the animals’ arousal threshold. In both sleep states, the octopuses needed a strong stimulus to evoke a behavioral response, researchers found.
The alternating sleep states seemed similar to human sleep, despite the enormous evolutionary distance between cephalopods and vertebrates, whose lineages diverged around 500 million years ago, according to first author and graduate student Sylvia Medeiros of the Brain Institute of the Federal University of Rio Grande do Norte, Brazil.
“If in fact two different sleep states evolved twice independently in vertebrates and invertebrates, what are the essential evolutionary pressures shaping this physiological process?” Medeiros said. “The independent evolution in cephalopods of an ‘active sleep’ analogous to vertebrate REM sleep may reflect an emerging property common to centralized nervous systems that reach a certain complexity.”
Intriguingly, the findings raise the possibility that octopuses experience something similar to dreaming.
“It is not possible to affirm that they are dreaming because they cannot tell us that, but our results suggest that during ‘active sleep’ the octopus might experience a state analogous to REM sleep, which is the state during which humans dream the most,” explains Medeiros.
If octopuses dream, it is unlikely that they experience complex symbolic plots like humans do, Medeiros points out. Considering that the “active sleep” stage typically lasts no longer than a minute, their dreams are more likely similar to short videoclips or even GIFs.
In future studies, researchers hope to record neural data from cephalopods to better understand what happens when they sleep. They also intend to probe the role of sleep in the octopuses’ metabolisms, thinking and learning while answering such questions as whether octopuses have nightmares and if their dreams are inscribed on their dynamic skin patterns.
“It is tempting to speculate that, just like in humans, dreaming in the octopus may help to adapt to environmental challenges and promote learning,” Ribeiro says.
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