When awake, octopuses change into a vibrant array of colors to blend into their surroundings and avoid predators. When asleep, octopuses curl themselves up and turn a white-grey color, but moments later, their bodies ripple into various hues as their muscles, suckers, and eight arms twitch. The color changes and movements suggest that octopuses may experience two sleep stages similar to mammals, called "quiet sleep" and "active sleep," reports Thomas Ling for BBC Science Focus. The study was published this week in iScience.
To find out if the changes in color while octopuses sleep are a result of dreaming, neuroscientist Sidarta Ribeiro of Brazil's Federal University of Rio Grande do Norte (UFRN) and his colleagues filmed four common octopuses (Octopus vulgaris) in laboratory tanks while they slept, reports Rodrigo Pérez Ortega for Science. During quiet sleep, the octopuses' pupils narrowed, their bodies became very pale, and they hardly moved, except for slightly swaying the tips of their arms, reports Nicoletta Lanese for Live Science. Following 30 to 40 minutes after initial quiet sleep, the octopuses dramatically shifted from pale white to shades of rusty orange. The octopuses also twitched their muscles, moved their eyes, and increased their ventilation rates, Live Science reports. The active sleep only lasted about 40 seconds but repeated after 30 to 40 more minutes of quiet sleep, Science reports. These patterns are similar to how mammals experience rapid eye movement (REM) sleep.
Humans usually dream while in REM sleep, and during this time, the eyes scurry under closed eyelids. Researchers suspect REM sleep is a way for mammals to store long-term memories and remove waste from the brain, reports Science.
"It is not possible to affirm that octopuses dream because they cannot tell us that, but our results suggest that during 'Active sleep' the octopus experiences a state analogous to REM sleep, which is the state during which humans dream the most," wrote study authors Ribeiro and Sylvia Medeiros, a cephalopod neuroscientist at UFRN, to CNN's Katie Hunt in an email.
To confirm the octopuses were genuinely asleep and not in a state of quiet alertness, the researchers had to tested their "arousal threshold," which is the time it takes to react to specific stimuli, reports Donna Lu for New Scientist. The scientists played videos of crabs outside the tanks to try to get the octopuses' attention, but none of them responded and were indeed fast asleep.
Octopuses in previous studies and experiments have exhibited sophisticated cognitive abilities and remarkable problem-solving skills, such as solving mazes, completing tasks to get treats, and even unscrewing containers, Lisa Hendry wrote for the Natural History Museum in 2017.
Given the advanced nature of their cognition, it's certainly possible they could be briefly dreaming during active sleep. To confirm this hunch, however, scientists need to collect neural recordings using electrodes while the octopuses sleep, which may be a challenge as the cephalopods tend to remove anything placed on their bodies, Live Science reports.
"If octopuses indeed dream, it is unlikely that they experience complex symbolic plots like we do," the study authors told CNN in an email. "'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 video clips, or even gifs."
The researchers plan to continue studying octopus sleep. Next, they'd like to investigate how lack of sleep affects an octopus's performance when trying to solve and complete various tasks, Science reports, just sleeplessness negatively affects humans' ability to function.