Fruit flies, octopus, birds and humans don’t seem to have much in common. Some live on land, others are aquatic. Some fly, while others are terrestrial. Some are vertebrates, others do not have a backbone. These creatures evolved separately, and their common ancestors are far, far down the evolutionary chain. But they can share one basic characteristic: they dream.
Almost all creatures sleep, although there is some debate as to whether single-celled organisms like Paramecium do. But nobody really knows Why. For years, researchers have talked about theories that sleep helps Memory, growth and learning – and it is clear that humans need sleep to work properly – but there isn’t much else that is fully understood. “Sleep is that big black box,” says Marcos Frank, a neuroscientist at Washington State University. Frank compares sleep to a mysterious organ: It is clear that it exists and is vital for animal health, but its exact function and the mechanisms that control it are still unknown.
It is even more mystifying that some species seem to have only one state of sleep, during which their brains are relatively calm, while others seem to experience two types, a calm phase and an active state. In humans, the period when the brain lights up with activity is called REM sleep (REM). It is when we dream and when we are the most difficult to wake up.
For a long time, scientists had not observed this deeper and more active phase of sleep 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 lizards. Then in 2019, the state was described in cuttlefish, and last March, a team of Brazilian scientists published a paper in iScience the identifier in octopus. Cephalopods like these evolved eons before the rise of creatures that would have shared a lineage with birds and humans. “There’s no way there’s a common ancestor there,” Frank says. Now scientists are wondering if this state of sleep is more common than they initially thought, or if it developed in different species at different times, the way wings and flight appeared 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 encode it could help scientists understand what function dreams serve the central nervous system and why sleep is important. “What does sleep do for animals?” asks Sidarta Ribeiro, co-author of the journal and director of the Brain Institute at the Federal University of Rio Grande do Norte.
The first step in studying how animals sleep is knowing when they are actually sleeping. It’s more complicated than it looks. “Imagine you were on Mars and found an organism,” Frank says. “How would you know if he was sleeping or not?”
For mammals, scientists could implant electrodes in their brains to track how their neurons fire. But octopuses have a highly distributed central nervous system. Instead of concentrating control of their nervous system in a single brain, they have eight ganglia in their arms that often act independently.
Rather than using an invasive method such as attaching probes to determine the sleeping states of octopuses, scientists at the Ribeiro Institute studied some of their behavioral characteristics. Sylvia Medeiros, graduate student and lead author of the study, tested the animals’ arousal thresholds. Three of the four octopuses in the lab received a visual stimulus – a video of moving crabs. We had a vibratory stimulus, in the form of a light tapping on his reservoir. Medeiros wanted to see how quickly they responded to stimuli when awake. Then she tested them when they appeared inactive and measured their response rates. Slower reactions meant they were more sound asleep.