Why octopuses might dream in color
Nothing to do with AI. Everything to do with intelligence. We couldn't help ourselves.
In 2019, a documentary called Octopus: Making Contact showed footage of an octopus named Heidi asleep in an aquarium tank. As she slept, her skin cycled through colours and textures - pale, then dark, then mottled and bumpy - as if a story were playing across her body. The filmmaker, biologist David Scheel, offered a tentative interpretation: perhaps she was dreaming of hunting a crab.
The clip went viral. It also raised a surprisingly serious scientific question. Do octopuses dream? And if they do, what would that tell us about intelligence that evolved completely separately from our own?
A very different kind of mind
Octopuses are molluscs, related to snails and clams. Our last common ancestor with them lived more than 500 million years ago and was probably a simple worm-like animal. Everything impressive about the octopus brain evolved independently of ours - which makes them one of the best natural experiments in intelligence we have.
Where an octopus keeps its neurons. Roughly two-thirds sit in the arms rather than the central brain.
A common octopus has around 500 million neurons, a similar number to a dog. But the layout is unlike any mammal: most of those neurons are in the arms. Each arm can taste, touch and react with a degree of independence, coordinated by a central brain that issues higher-level instructions.
Their behaviour matches the hardware. Octopuses solve puzzles, open jars, escape tanks, recognise individual humans, and in the wild have been filmed carrying coconut shells to use as portable shelters - one of the clearest examples of tool use in an invertebrate.
Octopuses really do have two kinds of sleep
In 2021, researchers in Brazil published a study in iScience describing two distinct sleep states in octopuses. In quiet sleep, the animal is pale and still, with narrowed pupils. Periodically it switches into active sleep: a short burst, lasting around a minute, in which skin colour and texture change rapidly, the eyes and suckers twitch, and the body contracts.
In 2023, a team at the Okinawa Institute of Science and Technology published a study in Nature that went further. Recording brain activity in a small species, Octopus laqueus, they found that active sleep looks remarkably like wakefulness in the brain, recurs roughly every hour, and that the skin patterns displayed during it resemble the patterns the animals use when awake. When the researchers interrupted active sleep, the octopuses compensated by entering it sooner and more often.
The two sleep states described in octopuses.
A striking parallel: Humans also alternate between two sleep types, and most vivid dreaming happens during REM sleep, when brain activity resembles being awake. Octopuses appear to have arrived at a similar two-stage pattern by an entirely separate evolutionary route.
So are they dreaming?
Nobody can say for certain, because dreams are subjective experiences and an octopus cannot report one. What researchers can say is that active sleep has several features consistent with dreaming:
The brain behaves much as it does when awake.
The body displays patterns linked to waking behaviour, such as camouflage and hunting displays.
The state appears to be needed: when it is interrupted, the animal makes up for it.
A leading hypothesis is that, as in mammals, this state helps consolidate memories - replaying and organising what was learned while awake. The OIST team also suggested it might help maintain the large repertoire of skin patterns the animal relies on. On this view, Heidi's changing colours could be the outward trace of something like a replay. That remains an interpretation, not a proven fact.
The colour twist
Here is where the headline gets complicated. Octopus eyes contain only one type of light-sensitive photoreceptor. In animals, colour vision normally requires at least two, so by the standard definition octopuses should be colourblind.
And yet they camouflage against colourful backgrounds with remarkable accuracy. Scientists have proposed a few explanations, all still debated.
Competing explanations for how a "colourblind" animal matches colours.
So the most accurate version of the title is this: octopuses almost certainly display colour while they sleep, in a state that may be their version of dreaming. Whether they experience anything in colour is unknown - and given their eyes, it may be something quite unlike human colour vision.
Why this matters beyond octopuses
For a long time, complex sleep was thought to be a feature of mammals and birds. Finding a similar two-stage pattern in an animal so distantly related suggests that sleep - and perhaps something like dreaming - may be a common solution to the problems any complex brain faces, such as storing and organising what it has learned.
That is also why the octopus is interesting to anyone thinking about intelligence in general, artificial intelligence included. It shows that capable minds can be built from very different architectures. Distributed rather than centralised, short-lived rather than long-lived, solitary rather than social - and still clever enough to recognise a face, solve a puzzle and, perhaps, replay its day in colour while it sleeps.
Key facts
About 500 million neurons, roughly two-thirds of them in the arms.
Two sleep states: quiet sleep, and short bursts of active sleep with rapid colour changes.
In active sleep, brain activity resembles waking, much like human REM sleep.
Octopus eyes have a single photoreceptor type, so how they perceive colour is still an open question.
Whether octopuses dream cannot be proven - but the evidence makes it a serious scientific possibility.