Most animals are asleep, although we still don’t quite know why. Scientists know that some animals – such as mammals, birds, and a few reptile species – have a stage called ‘active’ sleep, or what is known as REM (rapid eye movement) sleep. A new study, published March 25 in the open access journal iScience, sought to find out whether this “active” sleep phase also occurs in dormant octopuses, which are about as different from humans as you can get.
“The motivation to study the octopus is to see if an animal is so far from us, but has a very complex nervous system and very refined behavior – whether this animal can also actively sleep,” said senior author Sidarta Ribeiro, director. from the Brain Institute of the Federal University of Rio Grande do Norte, Brazil.
The study’s findings suggest they do. “We found that the octopus has two sleeping positions, one quiet and one active, just like humans,” says Ribeiro.
“We are in the midst of a revolution to understand what sleep does for us, and for organisms in general. That’s why this article is so cool, ”said Marcos Frank, a neuroscientist at Washington State University who knows Ribeiro but was not involved in the study. Frank has worked on similar questions in squid. “Evidence has been gathered over the past twenty years that sleep states similar to dreaming sleep in humans can occur in other organisms that have evolved independently of us.”
Led by Sylvia Lima de Souza Medeiros, a graduate student at the Brain Institute, the team of researchers studied four adult Octopus insularis, a species that lives off Brazil’s north coast. After capturing the animals and acclimating them in a lab setting, the team recorded them using computational methods to determine what happened in the different sleep positions.
They found a “calm” state, which was already known – in which the octopus is “completely pale, almost completely immobile, with very soft, sweet, smooth movements, the eyes are closed, the pupils are closed,” says Ribeiro.
Interestingly, the team noted that the condition of the marine animals seemed to change about every half hour: their eyes, arms, and suction cups began to move freely. And along with this movement came shifts in skin tone and texture. “The colors of the skin will change drastically.”
In 2019, a viral clip from a PBS documentary showed an octopus, Heidi, twitching, changing colors, and seemingly dreaming. Scientists at the time were cautious given the lack of data showing octopuses have REM-like sleep stages. In the iScience study, the researchers stimulated them with visuals and vibrations of scratching and measured their response to demonstrate whether these octopuses actually slept. The delayed response of the animals or the total lack of feedback suggested that they did indeed sleep during both the “quiet” and the “active” stages.
The authors don’t know if these octopuses were dreaming in the iScience study; even in humans, dreams are difficult to study. But because the “active” sleep state in the octopuses usually lasted only a minute, “if dreaming occurs during this state, it should look more like small video clips, or even gifs,” explains lead author Medeiros. in a statement.
A limitation of the study, Ribeiro says, is that they only studied four animals (which was “already a lot of work”). Furthermore, the best way to measure brain activity is by means of an electroencephalography, or an EEG, which requires the placement or implantation of electrodes temporarily on the head. But that’s hard to do with an octopus, whose head is smooth and boneless, and which tends to pick out unwanted accessories. Thus, the team was unable to measure brain activity that could further confirm their findings. “It is also a limitation in our study,” Frank notes.
Anytime a trait evolves independently, such as the eye or the wing, “it’s really exciting because it really tells you something about why that adaptation happened in the first place,” he says. “In the case of sleep, it’s extremely exciting because we don’t really understand the function of sleep.” The fact that this REM-like experience also occurs in octopuses, says Frank, “indicates that this state must do something important” – and understanding it could help unravel the mystery of why we sleep.