Dreaming, a complex state associated with vivid sensory experiences, has long fascinated researchers studying the sleeping brain. Comparative neuroscience seeks to determine if this state is exclusive to humans or shared across the mammalian class. The scientific answer lies in identifying the specific, measurable neurological state associated with most human dreaming and tracing its presence throughout the animal kingdom. Examining these distinct physiological markers allows scientists to explore the universality of this active sleep phase.
The Physiological Markers of Dreaming
Scientists distinguish between two primary sleep states: non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. NREM sleep is characterized by slow, high-amplitude brain waves and reduced physiological activity. REM sleep is the state most closely linked to vivid dreaming in humans. It is often called paradoxical sleep because the brain appears active, similar to a waking state, while the body is profoundly relaxed.
This paradoxical state is defined by a unique cluster of measurable physiological signals. Electroencephalogram (EEG) recordings show fast, low-voltage, desynchronized brain waves, resembling the electrical activity of an awake brain. Simultaneously, the body experiences a near-complete loss of skeletal muscle tone, known as atonia. This temporary paralysis is actively generated by inhibitory circuits in the brainstem, preventing the animal from physically acting out its dreams.
The signature feature that gives the state its name is the rapid, random movement of the eyes beneath the closed lids. Other physiological markers include intermittent muscle twitches, particularly in the face and limbs, and fluctuations in autonomic functions, such as breathing and heart rate. By monitoring these four traits—brain waves, muscle tone, eye movement, and autonomic function—researchers can objectively determine when a mammal enters the phase analogous to human dreaming.
Comparative Evidence Across Mammalian Species
While the capacity for REM sleep is widespread, its manifestation is not strictly universal across all mammalian species. The majority of terrestrial mammals, ranging from rodents and cats to primates, exhibit clear, cyclical patterns of REM sleep. The duration and intensity of this state vary widely and correlate strongly with an animal’s maturity at birth.
Mammals born in an altricial, or highly undeveloped, state—such as human infants, kittens, and rat pups—spend the largest proportion of their early lives in REM sleep. This high volume of active sleep declines as the animal matures. These species tend to maintain higher adult REM sleep percentages compared to precocial mammals, which are relatively independent soon after birth.
The most significant exceptions to the universality of REM sleep are found among marine mammals and evolutionary outliers. Cetaceans, including dolphins and whales, are not known to exhibit the classical REM state. Full muscle atonia would be lethal for these animals, as they must surface regularly to breathe. Instead, they rely on unihemispheric slow-wave sleep, where only one half of the brain sleeps at a time, allowing for constant movement and vigilance.
The most ancient mammalian lineage, the monotremes (echidnas and the platypus), presents a modified picture of REM sleep. Early studies suggested the echidna lacked the state entirely, but later research confirmed that both species experience a version of REM sleep. In the platypus, this state occurs in high amounts, though the EEG pattern lacks the low-voltage, desynchronized activity seen in other mammals. This suggests that while the core mechanisms for active sleep are ancient, the specific physiological signatures have adapted differently across the mammalian tree.
The Proposed Functions of Mammalian Dreaming
The widespread presence of REM sleep across diverse mammalian groups implies that the state serves deeply rooted biological purposes. One supported theory concerns the role of REM sleep in brain development. The extensive time spent in this state by altricial newborns suggests it provides necessary internal stimulation for the maturation of the central nervous system. The characteristic muscle twitches observed during REM sleep may function to engage and develop sensorimotor pathways when the animal is otherwise inactive.
A major area of research focuses on REM sleep’s function in memory consolidation and learning. Scientists propose that it is important for integrating spatial, procedural, and emotional memories. During this active sleep phase, the brain may selectively prune weak or newly formed synapses while strengthening others. This process helps refine and organize newly acquired information.
The unique neurochemical environment of REM sleep, characterized by an abundance of acetylcholine and low levels of stress-related chemicals, suggests a role in emotional processing. It is hypothesized that this state provides a safe environment for the brain to process and regulate emotionally charged experiences. While the precise function of REM sleep remains debated, the evidence points toward its necessity for optimal cognitive function and early life brain organization.