Octopuses sleep in technicolour. Do they dream, too?

A new study shows that octopuses cycle between active and quiet sleep states, in a way similar to humans, raising the possibility that their brain behaviour in sleep is similar as well.

In mammals, sleep states are important for memory processing and brain development. Since octopuses are known for their high learning capacity and complex nervous systems, neuroscientist Sidarta Ribeiro wanted to see whether their brains required multiple sleep states as well.

"If you just investigate cognition in animals that have the same kind of brain, you never learn what is fundamental," Ribeiro told Quirks & Quarks host Bob McDonald. "So I said I should study the octopus, which has very sophisticated cognition but has a very different brain design."

The study was published in the journal iScience.

Technicolour sleep cycles

Ribeiro is with the Brain Institute of the Federal University of Rio Grande do Norte in Brazil. His team, including grad student Sylvia Medeiros, filmed several octopuses in laboratory tanks as they slept and analyzed the recordings. Previous research had established that the invertebrates display an array of colour patterns while they doze, but the team noticed that those patterns occurred in regular intervals.

During quiet sleep, the octopuses were pale and still. But after about 30 minutes of quiet sleep, a period of what seemed to be active sleep kicked in for around a minute.

"The animals had so many changes in the colours of the skin, and the texture. The eyes moved around a lot. The arms moved a little bit in circles, and they also moved all their tentacles," said Ribeiro.

To confirm the animals were sleeping, the researchers played videos of crabs and tapped the tank with a rubber hammer. The animals reacted quickly to this when they were awake, but responded very slowly during both active sleep and quiet sleep.

Do octopuses dream?

Active sleep in humans is known as rapid eye movement, or REM, sleep, and that's when most vivid dreams occur. But Ribeiro is cautious about whether octopuses dream. 

He did suggest, however, that if they were dreaming, one telltale sign of what they're dreaming about could be evident in the patterns on their skin. One frequent pattern that flashed during the animal's sleep was one called the half-and-half pattern, where the animal is half light and half dark. When octopuses are awake and active, this is commonly used as a courtship display.

"It is tempting to speculate that when they are quiet and doing the half-and-half pattern, maybe they are dreaming about one of those events like a courtship event. There's also the possibility that they are awake and performing this pattern for some reason that we don't understand," he said.

"We probably won't know it ever, because they cannot really tell us," he said.

Ribeiro, who self-funded this research, is hoping that it helps show Brazil can be a great place to do science if it's properly funded.

"Brazil is undergoing a severe science crisis, and the budget for science in Brazil is now 25 per cent of what it was ten years ago, and we are, of course, one of the countries most hit by the pandemic situation," he said. 

"I hope that we get out of this situation, and I want to raise attention to the problems that are going on in Brazil right now."

Produced and written by Amanda Buckiewicz.