Just like humans, sleeping octopuses alternate between active and quiet sleep stages.
When octopuses slumber, they move between quiet periods, interrupted by short bursts of frenzied activity where their arms and eyes twitch, their breathing rate quickens, and their skin flashes with vibrant colours.
Researchers from the Okinawa Institute for Science and Technology and the University of Washington investigated the brain activity and changing skin patterns of octopuses while awake and asleep, Publishing their findings in Nature.
For a long time, only vertebrates were known to cycle between two different sleep stages.
The research reveals similar patterns in octopuses, which have complex cognition but completely different brain structures to mammals.
“The fact that two-stage sleep has independently evolved in distantly related creatures, like octopuses, which have large but completely different brain structures from vertebrates, suggests that possessing an active, wake-like stage may be a general feature of complex cognition,” says author Dr. Leenoy Meshulam, from the University of Washington.
The research adds to a growing body of evidence suggesting humans, other vertebrates, cephalopods and arthropods cycle between active and quiet sleep stages.
To begin, the scientists checked whether the octopuses were truly asleep during this active period. They tested how the octopuses responded to a physical stimulus and found that when in both the quiet and active stage of sleep, the octopuses required stronger stimulation before reacting, compared to when they were awake.
The researchers found the brain activity of octopuses during the quiet sleep stage closely resembled brain wave patterns in mammal brains during non-REM sleep.
During octopus quiet sleep, the Octopus laqueus closed their eyes, adopted a flat resting posture and a uniformly white skin pattern.
Roughly once an hour, the octopuses entered an active sleep phase for around a minute. During this stage, the creatures’ brain activity very closely resembled their brain activity while awake, akin to REM sleep in humans.
During this phase, the octopuses also cycled through skin patterns they use to camouflage themselves when awake. This was accompanied by eye and body movements and increased breathing.
To capture skin changes, the researchers collected and analysed 1,743 hours of video from nine sleeping cephalopods. To measure brain activity the researchers attached probes for measuring electrophysiological recordings.
There are various reasons for the similarities between the octopuses active sleep and awake states the researchers say.
One theory suggests the octopuses may be practicing their skin patterns to improve their waking camouflage behaviour, or simply maintaining the pigment cells.
Another possibility is that the octopuses could be re-living and learning from their waking experiences, such as hunting or hiding from a predator, and reactivating the skin pattern associated with each experience.
In other words, they could be doing something similar to dreaming.
“In this sense, while humans can verbally report what kind of dreams they had only once they wake, the octopuses’ skin pattern acts as a visual readout of their brain activity during sleep,” says Professor Sam Reiter, from the Okinawa Institute for Science and Technology.
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