Research shows many animals experience REM sleep, the stage associated with dreaming. Here is what scientists have discovered about animal dreams.
The question of whether animals dream has captivated scientists for decades, and the evidence increasingly points to yes. If dreaming is a product of REM sleep, and REM sleep exists across a wide range of vertebrate species, then many animals experience something analogous to what we call dreaming. What they dream about remains an open question, but the neurological infrastructure for dreaming appears to be ancient and widespread.
REM sleep has been identified in all mammals studied to date, including dogs, cats, rats, horses, dolphins, and elephants. It has also been observed in birds. Research published in Science in 2016 identified REM-like states in reptiles, suggesting that the evolutionary origins of dreaming sleep may predate the mammal-bird split.
The defining characteristics of REM sleep: rapid eye movements, muscle atonia, and desynchronized brain waves resembling waking consciousness, are present across these species. This strongly implies they experience a state functionally similar to human dreaming.
MIT neuroscientist Matthew Wilson recorded hippocampal activity in rats while they ran a maze, then observed the same specific firing patterns during subsequent sleep. The rats were apparently replaying the maze experience, exactly as humans replay daily events in dreams.
Applying this to dogs: given that dogs have clear REM periods marked by twitching, paw movements, and muffled vocalizations, researchers believe dogs replay daily experiences. A dog twitching during sleep is probably, at some level, chasing the same squirrel it encountered that afternoon.
Research published in Nature in 2019 showed that zebrafish exhibit a sleep state with features resembling vertebrate REM sleep, including eye movements and brain activity patterns distinct from non-REM sleep. This extends the possibility of dreaming further down the evolutionary tree than previously thought.
For insects, the evidence is much weaker. Fruit flies show rest states with features of sleep, but whether these include anything analogous to REM dreaming remains deeply uncertain given their comparatively simple nervous systems.
Antti Revonsuo proposed that dreaming originally evolved as a threat simulation system, allowing animals to rehearse survival responses without real-world risk. Howard Eichenbaum's work on rats demonstrated that sleep-based replay strengthens learning and spatial memory.
Both functions confer survival advantages, explaining why dreaming sleep has been conserved across hundreds of millions of years of evolution. Animals likely dream about the same things their waking lives are dominated by: food, territory, social relationships, and threats.
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