Sidarta Ribeiro at the Federal University of Rio Grande do Norte in Natal, Brazil, and his colleagues have found that octopuses go through two distinct stages of sleep – active and passive. The researchers recorded four common octopuses (Octopus vulgaris) in the laboratory over several day and night periods, amassing more than 180 hours of footage.
During the day, the animals slept for more than half the time, says Ribeiro. “In quiet sleep they stay in the same position for long periods of time – very quiet, very pale the pupils closed – and breathe regularly in a very quiet matter,” he says.
This passive sleep was punctuated every 30 to 40 minutes by a brief period of active sleep, lasting 1 to 2 minutes. In this phase, the octopuses showed changes in body colour and texture, including the protrusion of fine bumps on their skin known as papillae. The animals’ eyes and arms also moved, their suckers contracting. “It’s clearly a very active state,” says Ribeiro.
The team tested whether the octopuses were truly asleep in this state by presenting them with a video of some crabs. “When we stimulated the animal with visual or vibratory stimuli, they did not react,” says Ribeiro – in marked contrast with their responses when awake.
A similar pattern of sleep occurs in birds and reptiles, and the researchers suggest that the active sleep state in octopuses might be analogous to REM sleep in mammals – the phase in which we dream the most.
“If the octopus is having something like a dream, it’s probably a very short behavioural sequence, it’s not a narrative,” says Ribeiro. “Whether they are having some sort of inner life with a narrative about themselves… we don’t know.”
“It would be very interesting to see what’s happening in their brains while they go from awake to quiet and then active sleep,” he says.