Fruit flies, octopuses, birds, and humans don’t seem to have much in common. Some live on land, others are aquatic. Some fly, while others are earthbound. Some are vertebrates, others lack backbones. These creatures evolved separately and their common ancestors are far, far back in the evolutionary chain. But they may share one fundamental feature: They dream.
Nearly all creatures sleep, though there’s some debate as to whether single-celled organisms like paramecium do. But no one really knows why. For years, researchers have bandied about theories that sleep helps with memory, growth, and learning—and it’s clear that humans need sleep to function properly—but there’s little else that’s well understood. “Sleep is this big black box,” says Marcos Frank, a neuroscientist at Washington State University. Frank likens sleep to a mysterious organ: It’s clear that it exists and is vital to animals’ health, but it’s exact function and the mechanisms that control it are still unknown.
It’s even more mystifying that some species appear to have only one sleep state, during which their brains are relatively quiet, while others seem to experience two kinds, a quiet phase and an active state. In humans, the period when the brain lights up with activity is called Rapid Eye Movement (REM) sleep. It’s when we dream and when we are the most difficult to wake up.
For a long time, scientists hadn’t observed this deeper, active sleep phase in amphibians or reptiles. So until recently, the theory was that it evolved later in history, via an ancestor shared by birds and animals. But in 2016 active sleep was recorded in lizards. Then in 2019, the state was described in cuttlefish, and this March, a team of scientists in Brazil published a paper in iScience identifying it in octopuses. Cephalopods like these evolved eons before the rise of creatures that would have shared a lineage with both birds and humans. “There’s no way there’s a common ancestor there,” says Frank. Now scientists are wondering if this sleep state is more common than they initially realized, or if it developed in different species at different times, the way wings and flight arose separately in insects, bats, and birds, a phenomenon called convergent evolution.
Understanding which selective pressures caused this adaptation and the preservation of the genes that code for it could help scientists understand what function dreaming serves for the central nervous system and why sleep is important at all. “What is sleep doing for animals?” asks Sidarta Ribeiro, a coauthor on the paper and the director of the Brain Institute at the Federal University of Rio Grande do Norte.
The first step in studying how animals sleep is to figure out when they are, in fact, asleep. This is more complicated than it sounds. “Imagine you were on Mars and you found an organism,” says Frank. “How would you know if it were asleep or not?”
For mammals, scientists might implant electrodes in their brains to track how their neurons are firing. But octopuses have a highly distributed central nervous system. Instead of concentrating control of their nervous system in one brain, they have eight ganglia in their arms that often act independently.
Rather than using an invasive method like attaching probes to determine the octopuses’ sleep states, scientists at Ribeiro’s institute studied a few of their behavioral characteristics. Sylvia Medeiros, a graduate student and the lead author on the study, tested the animals’ arousal thresholds. Three of the lab’s four octopuses were given a visual stimulus—a video of moving crabs. One got a vibratory stimulus, in a form of a light tapping on its tank. Medeiros wanted to see how quickly they responded to stimuli when they were awake. Then she tested them when they seemed inactive, and measured their response rates. Slower reactions meant they were more deeply asleep.