Octopuses Exhibit Active Sleep: A Fascinating Resemblance to Human REM Sleep

Octopuses, surprisingly, showcase two distinct sleep stages—quiet and active—bearing similarities to the rapid eye movement (REM) sleep observed in mammals. This active sleep stage in octopuses exhibits behaviors and neural patterns akin to wakefulness, potentially resembling the role of dreams in humans.

Similar to humans, octopuses undergo a transition between two sleep stages: a calm stage and an active stage reminiscent of REM sleep in mammals. Could this suggest that octopuses experience dreams?

During their sleep, octopuses have moments of tranquility interspersed with brief bursts of vigorous activity. Their eyes and arms twitch, their breathing accelerates, and their skin radiates with vibrant hues.

Scientists from the Okinawa Institute of Science and Technology (OIST), in collaboration with the University of Washington, have conducted a meticulous investigation of octopuses’ brain activity and skin patterns during this active sleep period, finding striking resemblances to awake neural activity and skin patterning. This wake-like activity also occurs during the rapid eye movement (REM) phase of sleep in mammals—the stage associated with most dreams.

Octopus laqueus during quiet sleep, appearing motionless and white. Active sleep with wake-like activity occurs approximately every hour. Credit: Keishu Asada (OIST)

Published on June 28 in the journal Nature, the study sheds light on the remarkable similarities in sleep behavior between octopuses and humans, offering captivating insights into the origin and purpose of sleep.

“All animals appear to experience some form of sleep, even simple organisms like jellyfish and fruit flies. However, for a long time, only vertebrates were known to cycle between two distinct sleep stages,” stated Professor Sam Reiter, the senior author and leader of the Computational Neuroethology Unit at OIST.

The exploration into octopuses’ sleep patterns unfolded through the visit of Dr. Leenoy Meshulam, a statistical physicist from the University of Washington, who spent three months as a guest in the OIST Theoretical Sciences Visiting Program.

“The fact that two-stage sleep has independently evolved in distantly related creatures like octopuses, which possess large yet completely distinct brain structures from vertebrates, suggests that having an active stage resembling wakefulness could be a common feature associated with complex cognition,” explained Dr. Meshulam.

During the initial stages, the scientists verified if the octopuses were genuinely asleep during their active period. They tested the octopuses’ response to physical stimuli and discovered that during both quiet and active sleep stages, the octopuses required stronger stimulation before reacting compared to their awake state. Additionally, the team observed that when they prevented the octopuses from sleeping or disturbed them during the active sleep phase, the octopuses entered the active sleep stage more frequently and sooner.

“This compensatory behavior solidifies the active stage as an essential component of sleep, necessary for octopuses to function properly,” noted Aditi Pophale, co-first author of the study and a PhD student at OIST.

The researchers also delved into the octopuses’ brain activity during wakefulness and sleep. During the quiet sleep stage, the scientists observed distinct brain wave patterns closely resembling certain waveforms observed during non-REM sleep in mammalian brains, known as sleep spindles. While the precise function of these waveforms remains unclear even in humans, scientists believe they aid in consolidating memories. Using a state-of-the-art microscope constructed by co-first author Dr. Tomoyuki Mano, the researchers determined that these sleep spindle-like waves occur in regions of the octopuses’ brains associated with learning and memory, implying a potentially similar function to humans.

Approximately once every hour, the octopuses entered an active sleep phase lasting around a minute. During this stage, their brain activity strikingly resembled their brain activity during wakefulness, much like REM sleep in humans.

The research team also captured and analyzed the changing skin patterns of the octopuses in ultra-high 8K resolution while they were awake and asleep.

“By recording in such high resolution, we can observe how each individual pigmented cell behaves and contributes to the overall skin pattern,” remarked Dr. Meshulam. “This could aid in creating simplified models of skin patterns to understand the fundamental principles of waking and sleeping patterning behavior.”

When awake, octopuses manipulate thousands of minute, pigmented cells in their skin, resulting in a wide range of distinct skin patterns. These patterns serve various purposes, including camouflage in different environments and social or defensive displays, such as deterring predators and communication among octopuses. During active sleep, the researchers observed that the octopuses cycled through these same skin patterns.

The resemblances between active sleep and wakefulness in octopuses could be explained by multiple theories, according to the scientists. One theory suggests that octopuses may practice their skin patterns during sleep to enhance their camouflage abilities while awake or maintain the functionality of pigment cells.

Another intriguing idea posits that octopuses might relive and learn from their experiences during wakefulness, such as hunting or evading predators, by reactivating the associated skin patterns. In other words, they could be engaging in a process akin to dreaming.

“In this sense, while humans can verbally recount their dreams only upon awakening, the octopuses’ skin pattern serves as a visual representation of their brain activity during sleep,” elaborated Professor Reiter.

He further added, “Currently, we are uncertain which of these explanations, if any, could be correct. Further investigations hold immense interest for us.”

Reference: “Wake-like skin patterning and neural activity during octopus sleep” by Aditi Pophale, Kazumichi Shimizu, Tomoyuki Mano, Teresa L. Iglesias, Kerry Martin, Makoto Hiroi, Keishu Asada, Paulette García Andaluz, Thi Thu Van Dinh, Leenoy Meshulam, and Sam Reiter, 28 June 2023, Nature.
DOI: 10.1038/s41586-023-06203-4

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More about Octopuses, REM sleep, dreaming

• Nature: “Wake-like skin patterning and neural activity during octopus sleep”
• Okinawa Institute of Science and Technology (OIST)
• University of Washington
• Computational Neuroethology Unit at OIST