Octopuses change colour while they sleep because they are dreaming, study finds

An octopus off the coast of south east Africa

Credit: Gabriel Barathieu 

The mystery of why octopuses change colour while they sleep has been solved — they are dreaming, a study has revealed.

When they are in a dream-like state called "active sleep", their eyes flicker, suckers contract, body twitches, and skin tone fluctuates, scientists found.

Becoming a different colour can act as camouflage so they can feed without being disturbed, or be a warning that predators are close by.

This suggests that the eight-limbed creatures may be blissfully dreaming that they are catching prey such as crabs, or having a nightmare that whales are on the prowl.

In contrast, when octopuses are in so-called "quiet sleep" with no dreaming, their eyes are closed, any movements are soft and slow, and their skin remains a pale colour.

Octopuses have special pigment cells called chromatophores in their skin. By controlling the size of the cells they can vary their colour and even create changing patterns.  

The research was carried out by scientists from the Federal University of Rio Grande do Norte in Brazil.

Sylvia Medeiros, from the university’s Brain Institute, said: "It is not possible to affirm that they are dreaming because they cannot tell us that, but our results suggest that during ‘active sleep’ the octopus might experience a state analogous to REM sleep, which is the state during which humans dream the most.

"If octopuses indeed dream, it is unlikely that they experience complex symbolic plots like we do.

"Active sleep in the octopus has a very short duration — typically from a few seconds to one minute. If during this state there is any dreaming going on, it should be more like small videoclips, or even gifs."

Scientists studied one male and three female Octopus insularis, a tropical species from the Western Atlantic, in a laboratory. They were allowed to settle into the artificial setting for 10 days before observations of their sleeping behaviour began.

To measure the depth of the octopuses’ sleep, the scientists used two tests. The first involved putting a computer screen outside the tank and playing a video of live crabs moving around. For the second test, the tank was vibrated.

When the octopuses were believed to be asleep, in either active or quiet state, they needed a very strong stimulus to prompt a response.

Scientists found that "active sleep" periods, when the octopuses may be dreaming, came in bursts of up to 60 seconds after at least six minutes of "quiet sleep".

In future studies, they suggest recording the brain activity of octopuses to see if it helps understand these two sleeping states.

The study was published in the journal iScience.