Octopuses, those fascinating and enigmatic cephalopods, may experience REM sleep or something akin to it. A recent study led by the Okinawa Institute of Science and Technology has confirmed the existence of two alternating sleep states in octopuses: a quiet one and an active one that resembles wakefulness. The neural architecture of octopuses is vastly different from that of other organisms, but they have similar brain structures that enable them to exhibit incredible problem-solving skills.
Furthermore, scientists previously thought that only vertebrate animals undergo the rapid eye movement (REM) phase of the sleep cycle. However, researchers found similar activity in the cephalopod family. In 2012, scientists observed cuttlefish display a state that is possibly analogous to REM sleep, characterized by rapid eye movements, changes in body coloration, and arm twitching. In 2021, another team observed color changes in sleeping octopuses of the species Octopus insularis and found alternating sleep states: a quiescent state characterized by washed-out colorlessness and an active state with vivid flickering color changes, cycling through at 30- to 40-minute intervals.
In the recent study, researchers simultaneously studied the coloration changes and neural activity of octopuses in their sleep versus wake states. The team used computational analysis of skin patterning and high-density electrophysiological recordings from the central brain of sleeping and awake octopuses of the species Octopus laqueus. They discovered that, during each sleep phase, their brains showed different kinds of activity. In quiet sleep, they had short bursts of neural activity, very similar to what are known as "sleep spindles" that show up as spikes in an electroencephalogram in non-REM sleep in vertebrates. During the cephalopods' active sleep, their brain activity was very similar to their brain activity while awake – similar to an REM sleep state in vertebrates.
Interestingly, during their wake-like sleep phase, the octopuses cycled through the different patterns of coloring that they display for communication, camouflage, and defense. The researchers believe that these patterns could indicate that the octopuses are reliving their waking experiences while they sleep – something very similar to dreaming. It's still not clear what purpose these activity bursts serve in humans, though scientists believe they have something to do with cementing memories. In the case of octopuses, the researchers traced the origin of this activity in quiet sleep to the memory and learning center of their brain, suggesting it could play a similar role. This is a fascinating discovery that provides insight into the evolution and function of sleep. As statistical physicist Leenoy Meshulam from the University of Washington suggests, "possessing an active, wake-like stage may be a general feature of complex cognition." It's incredible to see how such vastly different organisms can share similar traits and behaviors, giving us a glimpse into the mysteries of our own consciousness.