“Whether swimming in the water, flying in the sky, or running on land, we can observe that almost all animals exhibit signs of sleep,” wrote Aristotle in his treatise On Sleep and Sleeplessness.
But do other animals actually dream? The ancient Greek philosopher had his own thoughts on the matter.
In The History of Animals, he noted, “It seems that not only humans dream, but also horses, dogs, cows, sheep, and goats. All four-legged, live-bearing animals dream. Dogs even bark while asleep, suggesting they are dreaming.” Though his approach was rudimentary, Aristotle’s conclusions weren’t entirely off base.
It is impossible to directly ask animals whether they dream, but evidence of their dreams can still be observed. Scientists have developed two primary methods to tackle this seemingly impossible task: observing physical behaviors during different sleep phases and analyzing brain activity during sleep to compare it with human patterns.
In the 1960s, researchers began exploring ways to understand animals’ dreams. At the time, medical journals reported sporadic instances of people exhibiting physical movements in their dreams. This discovery was intriguing, as muscles are typically paralyzed during the rapid eye movement (REM) sleep phase. Researchers realized that guiding animals into a similar state could provide insight into their dreams. In 1965, French scientists Michel Jouvet and J. F. Delorme discovered that removing a section of the brainstem called the “pons” in cats prevented their muscles from becoming paralyzed during REM sleep. They termed this condition “REM sleep without atonia” (REM-A). Cats in this state didn’t remain still while sleeping but instead walked around and displayed aggressive behaviors. This suggested they were dreaming about activities from their waking hours.
Subsequent studies uncovered similar phenomena. Veterinary neurologist Adrian Morrison commented on this research, noting that REM-A cats turned their heads in response to imagined stimuli and even displayed predatory behaviors resembling the act of hunting mice in their dreams. Dogs have also been observed exhibiting similar dreamlike activities.
Humans sometimes act out their dreams too, a condition known as REM sleep behavior disorder. According to the International Classification of Sleep Disorders (ICSD), common clinical manifestations include punching, kicking, jumping, or running out of bed during sleep, often aligning with dream content. The ICSD also notes that both individuals with this condition and their bed partners frequently sustain injuries.
Physical movement is not the only way to study dreams. Researchers now use humane methods to monitor the neural and chemical activity of animals’ brains during sleep. In 2007, scientists at MIT, Kenway Louis and Matthew Wilson, recorded neuronal activity in the hippocampus of rats, a brain region involved in memory formation and encoding. First, they tracked brain activity as rats navigated a maze, then observed identical neural firing patterns while the rats slept. The researchers found that rats exhibited the same neuronal firing sequences during REM sleep as they did while running through the maze. The results were so clear that the scientists could even pinpoint the rats’ positions in the dream maze, aligning them with real-life locations.
Further research has uncovered similar findings in birds. Biologists Amish Dave and Daniel Margoliash at the University of Chicago studied the brains of zebra finches and found intriguing patterns. These birds do not naturally sing well but must learn to do so. While awake, neurons in their forebrain’s robust nucleus of the archistriatum (RA) fire in patterns corresponding to the rhythm of their songs. By studying these firing patterns, researchers could identify the notes the birds were singing and even reconstruct entire melodies. When the birds slept, the RA exhibited similar firing patterns, indicating that the finches might be practicing their songs in their dreams.
Can the behaviors observed in cats during experiments truly be defined as dreams? Do rats subjectively perceive themselves running through mazes while asleep? Are songbirds aware they are singing in their sleep? These questions are as difficult to answer as those concerning the nature of consciousness itself. Humans, too, often do not realize they are dreaming until they wake up and reflect on their experiences. Do zebra finches recognize upon waking that they were dreaming? Can they distinguish reality from dreams?
Within reasonable limits, it can be concluded that the physiological and behavioral traits of dreaming observed in humans also occur in cats, rats, birds, and other animals. However, what the experience of dreaming truly feels like for these animals remains an unresolved mystery.
