For the first time, scientists have shown that the color-changing patterns observed in octopuses as they sleep are characteristic of two major sleep phases: one active and one quiet.
The study, published March 25 in iScience, suggests that these two sleep states may even make it possible for octopuses to dream. The findings also have wider implications for the evolution of sleep.
Sleep is nearly ubiquitous among animals. However, for many species, the degree to which animals experience different sleep states is unknown. Many invertebrates experience a single quiet sleep phase with no active phase. Humans experience three stages of sleep, including an active stage called REM sleep, which is when people dream.
In an interview with The Academic Times, senior author Sidarta Ribeiro, a neuroscience professor at Federal University of Rio Grande do Norte, explained that varying sleep states have been found to occur in animals as small as a fruit fly.
Ribeiro and his team wondered if the octopus could experience stages of sleep as well, both because they are among the most intelligent invertebrates — for example, a recent study showed they can feel pain both physically and abstractly — and because a REM-like sleep state was recently discovered in cuttlefish, a relative of the octopus. Ribeiro suggests the variety in animals that experience sleep states may relate to brain complexity, not size.
"There's something that is similar in those groups of animals that we may consider, which is the existence of a complex brain with many parts," he said. "In the case of the octopus and in vertebrates, big brains, and in [fruit flies] a small brain, but still a very complex brain."
Sleep studies in humans and other vertebrates usually rely on electrodes placed on the body to monitor brain and muscle activity. However, this is all but impossible to do when studying an animal as flexible as an octopus.
"They have no bones. There's nothing you can attach a screw to or paste anything to," Ribeiro said. "What most people do — and this is what we did — is observe their behaviors carefully, monitor them properly and do a lot of image analysis to extract features that are relevant to tell the story of what's going on inside the animal from the outside."
The researchers captured hours of footage of octopuses in the lab and monitored their behavior and appearance while awake and asleep. They found that after a longer period of quiet sleep, characterized by pale coloration, contracted pupils and very little movement, the octopuses would enter a brief active phase where they changed their color wildly while contracting their muscles.
These states alternated in a synchronized cycle of around 30 to 40 minutes.
"For me, the most shocking thing was the periodicity. Rats will go through REM sleep frequently, but not regularly," Ribeiro said. "I did expect the octopus to have active sleep, but I did not expect it to be so regular."
The researchers believe that octopuses' active sleep phase may allow them to have dreams, comparable to how humans dream during REM sleep.
"If octopuses indeed dream, it is unlikely that they experience complex symbolic plots like we do," said first author Sylvia Medeiros, a graduate student in Ribeiro's lab. "'Active sleep' in the octopus has a very short duration — typically from a few seconds to one minute. If during this state there is any dreaming going on, it should be more like small videoclips, or even GIFs," the short animated pictures.
While the study informs scientists' knowledge of octopuses, it does leave unanswered questions about the evolutionary forces shaping sleep cycles.
"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 asked. "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."
For now, the researchers hope this finding will encourage people to consider octopuses and other invertebrates in a new and better light.
"This is one more study that adds evidence towards octopuses being very, very smart and sophisticated," Ribeiro said. "In some countries they get the same protections as the higher primates, but in most countries, they don't. I think this is just one more study pointing to, to our need to respect them, investigate them and protect them."
The study, "Cyclic alternation of quiet and active sleep states in the octopus," published March 25 in iScience, was authored by Sylvia Lima de Souza Medeiros, Mizziara Marlen Matias de Paiva, Paulo H. Lopes, Wilfredo Blanco, Jaime Bruno Cirne de Oliveira, Inácio Gomes Medeiros, Eduardo Bouth Sequerra, Sandro de Souza and Sidarta Ribeiro, Federal University of Rio Grande do Norte; and Françoise Dantas de Lima and Tatiana Silva Leite, University of Santa Catarina.