Octopuses have alternating periods of “quiet” and “active” sleep that make their rest similar to that of mammals, despite being separated by more than 500 million years of evolution.
When octopuses snooze on the seafloor, their skin sometimes pulses with an array of colors, and at other times, they become pale and plain. These alternating patterns mark two distinct stages of the octopus sleep cycle, a small study suggests.
During “active sleep,” when an octopus’s skin ripples with dazzling colors, the cephalopod may experience something similar to our rapid-eye-movement (REM) sleep, the authors wrote in the study, published March 25 in the journal iScience. Humans do most of their dreaming during REM sleep, but for now, we don’t know if octopuses also have dreams in their sleep — or what they’d dream about if they did.
Octopuses change color using chromatophores, or specialized pigment organs that expand and contract under the skin, altering the colors and patterns on its surface. While awake, octopuses can change color to blend in with their surrounding environment, but it’s unknown why the animals continue to shift color while at rest, and few studies of octopuses sleep have explored the phenomenon.
To confirm that an animal is truly asleep, scientists test its “arousal threshold,” meaning the amount of time it takes the creature to react to a stimulus. For example, while awake, an octopus will quickly react to physical vibrations of its tank or to videos of scuttling crabs played just outside the glass. A sleeping octopus will take far longer to react, or may not respond at all, since it must first be roused from slumber.
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Carrie Albertin, who studies octopuses at the Marine Biological Laboratory in Woods Hole, Mass., says these creatures spend a lot of time in their dens and it certainly can look like they’re sleeping and even dreaming.
“I think as you watch these animals, it’s really hard to deny that something is going on, but it’s really important to actually quantify it and do the study and set up the study so that you can characterize it in a rigorous way,” says Albertin. “And that’s exactly what this group has done.”
She thinks this work is a good first step in characterizing the different stages of octopus sleep.
“I think it’s really important to study these sorts of questions in animals like octopuses and cuttlefish,” says Albertin, “because they are a separate example of the evolution of large brains. And so they are telling us something fundamental about what it is to have a large brain and what you need as part of that.”