Octopus and human sleep show striking parallels, including wake-like phase

The research team was led by Sam Reiter, who leads the Computational Neuroethology Unit at OIST. The team also comprises Dr. Leenoy Meshulam, a statistical physicist at the University of Washington, and co-first authors Aditi Pophale and Dr. Tomoyuki Mano from OIST.

"All animals seem to show some form of sleep, even simple animals like jellyfish and fruit flies. But for a long time, only vertebrates were known to cycle between two different sleep stages," said Reiter in a press release.

Brain activity & response to stimuli

The team began by checking the octopuses' response to a physical stimulus to determine whether they were really asleep. They discovered that a stronger stimulus was required to elicit a reaction in both the active and quiet stages compared to when the octopuses were awake.

The researchers also observed that preventing the octopuses from sleeping or disrupting them during active sleep caused the octopus to enter the active sleep stage earlier and more often. This compensatory behavior established that the active stage of sleep was essential for the octopuses to function well.

They further studied their brain activity, discovering that during quiet sleep, the octopuses exhibited brain waves similar to waveforms observed during non-REM sleep in mammals known as sleep spindles.

These sleep spindles are believed to aid in memory consolidation. Using a cutting-edge microscope developed by Mano, the team concluded that these spindle-like waves occur in brain regions associated with learning and memory in octopuses, suggesting a similar function to humans.

They also noted that once every hour, the octopuses went into an active sleep phase for roughly one minute, in which their brain activity mimicked their brain activity while awake, exactly as REM sleep does in humans.

Observing skin patterns in 8K

Octopuses possess the ability to control pigment cells in their skin, enabling them to create various skin patterns for camouflage, social interactions, and predator deterrence. Scientists have previously observed frenzied shifts in color suggestive of dreams in octopuses. 

The research team decided to observe the change in color using an ultra-high 8K resolution camera.

They found that the octopuses cycled through the same skin patterns they use when hiding from prey or camouflaging. 

According to the researchers, the similarities between awake states and active sleep in octopuses can be attributed to various possibilities. One theory suggests that octopuses may use active sleep to practice and refine their skin patterns for improved camouflage. Another exciting idea is that they may re-live and learn from waking experiences, akin to dreaming, by reactivating the associated skin patterns.

"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. We currently don't know which of these explanations, if any, could be correct. We are very interested in investigating this further," said Reiter.

The findings of the study are published in the journal Nature.