Two years ago a video of an octopus changing color as it slept went viral, with many people jumping to the conclusion the cephalopod was dreaming. The video's author stressed he is not a sleep biologist and this interpretation may not be right. Now, however, some animal neuroscientists have published their investigation of octopus sleep, and it looks like this time the amateurs were right – octopuses probably do something analogous to our dreams.
There is unfortunately no way we can know what crosses the mind of an animal while sleeping. Nevertheless, everyone who shares a house with a dog has seen the twitching moments they sometimes make as they sleep, commonly referred to as “dreaming of the hunt”. It was once thought Rapid Eye Movement or REM-sleep was restricted to mammals and birds, but bearded dragons have been shown to undergo something that involves similar brainwaves. Cuttlefish have also been reported to have two sleeping states, one of which may be equivalent to REM sleep.
"That led us to wonder whether we might see evidence of two sleep states in octopuses, too," Professor Sidarta Ribeiro of the Federal University of Rio Grande do Norte, Brazil said in a statement. The learning and problem-solving capacities of octopuses are legendary. As such distant relations, with short lifespans and receiving no care from others of their species, they represent an intelligence that has taken a very different path not only from ourselves, but from others we usually study.
Ribeiro confirmed octopuses do indeed have two sleep modes, referred to as “active” and “quiet” sleep. During the short (typically 41 seconds) bouts of active sleep four Octopus insularis were seen to change their skin color and texture, as they do when seeking to camouflage themselves or signal to others of their kind. They also twitched their muscles, contracting their suckers. Most reminiscent of human dreaming, the active-sleeping octopuses moved their eyes.
"What makes it more interesting is that this 'active sleep' mostly occurs after a long 'quiet sleep' – generally longer than 6 minutes – and that it has a characteristic periodicity," Ribeiro said. Cycles ran for 30-40 minutes in the subjects, but the fact the oldest and largest member of the set had a somewhat different cycle made the authors wonder if there might be changes with age.
Although active sleep might be mistaken for wakefulness by a casual observer, Ribeiro reports in iScience the octopuses did not respond to visual stimulus (or in one case vibrations) as they would when awake.
First author Sylvia Medeiros noted the similarity of the cycles to our own, “Despite the enormous evolutionary distance between cephalopods and vertebrates, with an early divergence of lineages around 500 million years ago.”
We don't have evidence of active sleep among all animals that share a common ancestry with us and octopuses, and indeed it seems some reptiles lack it. This makes it unlikely active sleeping evolved before vertebrates and molluscs diverged; instead probably evolved independently. If so, Medeiros asked, “What are the essential evolutionary pressures shaping this physiological process?"