The question of whether animals experience dreams has long captured human curiosity, often fueled by watching a twitching dog or cat. While a definitive answer remains elusive, modern science investigates the neurological underpinnings of sleep across the animal kingdom. The ability to dream is intrinsically tied to specific physiological states observed during sleep. Exploring these states shows that while restorative rest is nearly universal, the specific brain activity associated with vivid dreams is not.
Defining Animal Dreaming Through Sleep Cycles
Dreaming is defined by the presence of a distinct physiological state known as Rapid Eye Movement (REM) sleep. This stage is characterized by a specific suite of markers that researchers can measure objectively across different species. During REM sleep, the brain exhibits electrical activity patterns that closely resemble the low-amplitude, high-frequency activity seen in an awake state.
A second feature of this phase is muscle atonia, a temporary paralysis of the body’s skeletal muscles. This mechanism is thought to prevent the sleeper from physically acting out the mental activity occurring in the brain. The third marker, which gives the phase its name, is the rapid darting of the eyes beneath closed eyelids.
This combination of brain activity, muscle inhibition, and eye movement is the scientific standard used to infer a dream-like state in non-human animals.
Strongest Evidence: Dreaming in Mammals
Mammals demonstrate the strongest evidence for dreaming because their sleep architecture closely mirrors the human pattern of cycling between non-REM and REM sleep. Definitive proof of dream content was provided by research on rats, which were trained to run a circular maze while their brain activity was mapped.
During subsequent REM sleep, the specific pattern of neuronal firing in the hippocampus, the brain’s memory center, was reactivated. The neural signature replayed during sleep was so precise that researchers could reconstruct where in the maze the sleeping rat was dreaming it was. This suggests that the rats were re-processing and consolidating the day’s events, which is the functional equivalent of dreaming about a recent experience.
In another classic demonstration, scientists temporarily disabled the muscle atonia mechanism in cats. Without the normal paralysis, the sleeping cats began to physically perform complex, coordinated movements, such as stalking, pouncing, and arching their backs, behaviors consistent with acting out a dream narrative.
Anecdotal evidence from dog owners observing their pets’ muffled barks, leg twitches, and tail wags during sleep aligns with these physiological findings. The shared and similar sleep cycles across mammalian species provide the strongest foundation for the conclusion that many mammals also experience dreams.
Sleep States in Non-Mammalian Vertebrates
Non-mammalian vertebrates show that while the two distinct sleep states are not exclusive to mammals, they manifest differently. Birds, for instance, cycle through both non-REM and REM sleep, but their REM periods are extremely brief, often lasting only a few seconds at a time. This suggests that if birds dream, their dream states are highly fragmented and momentary compared to the longer, more sustained episodes experienced by mammals.
Reptiles, such as the Australian bearded dragon, also display evidence of non-REM and REM-like states, which suggests that the evolutionary origin of dual-stage sleep may be much older than previously assumed. In contrast, many aquatic mammals and certain bird species exhibit a unique pattern known as unihemispheric sleep. This adaptation allows one half of the brain to rest while the other half remains awake, enabling animals like dolphins and ducks to maintain vigilance or surface periodically to breathe.
While fish show periods of quiescence and reduced responsiveness that meet the behavioral definition of sleep, the neurological evidence for a clear, distinct REM phase is less consistent. Some fish, like the zebrafish, have demonstrated a REM sleep-like state, but the overall architecture is often simplified or structurally different from the mammalian model.
The variety of sleep architecture highlights that the brain processes underlying dream-like states have evolved along multiple, distinct paths.
Exploring Rest and Sleep in Invertebrates
To address whether all animals dream, researchers must look toward invertebrates, which lack the centralized brains characteristic of vertebrates. Sleep in these organisms is identified using behavioral criteria, such as reduced movement, a specific resting posture, and an increased threshold for arousal. For example, studies on the common fruit fly, Drosophila melanogaster, show that if deprived of rest, they will exhibit a homeostatic rebound, sleeping more to compensate for the lost time, which is a hallmark of true sleep.
More complex invertebrates, particularly cephalopods like octopuses and cuttlefish, offer evidence of complex states during rest. These animals have nervous systems and display an active sleep phase similar to REM sleep. During this active phase, a sleeping octopus will rapidly cycle through a sequence of camouflage patterns and skin texture changes.
Scientists speculate that these shifting colors may represent a visual replay of waking experiences, analogous to the imagery of a dream. While it remains impossible to confirm subjective experience, the observation of complex, coordinated physiological changes during rest in a species as evolutionarily distant as the octopus suggests that the potential for complex mental states during sleep may be more widespread than once believed. Ultimately, while sleep is a near-universal requirement for animals, the complex phenomenon of dreaming, as defined by REM sleep, is primarily confirmed in mammals, with structurally different equivalents found elsewhere.