Across the diverse branches of the animal kingdom, the mystery of sleep’s evolutionary origins has long captivated neuroscientists. A groundbreaking new study published in Nature Neuroscience upends existing paradigms by revealing a sleep-related brain rhythm conserved not just among mammals, but stretching far back to reptiles. By exploring neural activity spanning seven different species of lizards alongside humans, rats, and pigeons, researchers have unearthed evidence of an infraslow brain rhythm during sleep that appears fundamental across amniotes. This finding challenges long-held assumptions about the evolution and architecture of sleep states, suggesting a profound commonality among species separated by hundreds of millions of years.
The research team, led by Bergel, Schmidt, and Barrillot, employed advanced brain recording techniques to monitor infraslow rhythms—ultra-slow oscillations occurring at frequencies lower than traditional sleep waves—across these varied animals. In lizards, this rhythm was strikingly synchronized with physiological markers such as eye movements, muscle tone, as well as heart and breathing rates, weaving a complex tapestry of integrated sleep-related signals. In chameleons, the rhythm’s signature extended to skin brightness fluctuations, while in bearded dragons, it aligned with pulsatile changes in cerebrovascular volume, linking neural activity to cerebral blood flow dynamics during sleep. Remarkably, similar infraslow oscillations appeared during non-rapid eye movement (NREM) sleep in mice, underscoring the rhythm’s cross-species conservation.
This infraslow frequency band sleeps under the radar of classical neuroscience, traditionally overshadowed by more familiar rhythms like delta waves, spindles, and gamma oscillations. Yet, its pervasive presence across taxa hints at a fundamental physiological role. Prior work primarily focused on mammalian models, often neglecting reptiles whose simpler brain architecture might hold clues to sleep’s evolutionary trajectory. By bridging this gap, the current study illuminates how basic neural rhythms underlying sleep may have been preserved or adapted across evolutionary timescales, underpinning behavioral and homeostatic functions in vastly different brains.
One of the most striking aspects of this study is the discovery of how tightly linked these slow rhythms are to multifaceted physiological processes during sleep. In lizards, for example, eye movements and muscle tone, typically associated with transitions between sleep stages, were entrained to these infraslow oscillations. This finding not only underscores the integrative nature of these rhythms but also suggests these oscillations may coordinate systemic changes necessary for sleep maintenance and restoration. Such coupling points to a mechanism for synchronizing brain and bodily states, which may have been evolutionarily prioritized due to its survival advantages.
Equally revelatory is the demonstration that these brain rhythms correspond with cerebrovascular volume changes in both reptiles and mammals. The bearded dragon’s pulsatile cerebral blood volume fluctuations echo similar phenomena detected in the rodent brain during NREM sleep. This cross-species vascular coupling suggests an ancient neurovascular regulatory mechanism facilitating metabolic waste clearance or nutrient delivery during sleep. Should these dynamics prove ubiquitous, they would advance the glymphatic hypothesis, which posits the importance of sleep in clearing toxic byproducts from the brain, linking vascular oscillations to sleep’s restorative biology.
The implications for evolutionary biology are profound. Amniotes, the clade encompassing mammals, birds, and reptiles, diverged around 310 million years ago. Identifying a conserved infraslow sleep rhythm implies that fundamental aspects of sleep physiology may predate the emergence of mammals by an extensive evolutionary margin. This challenges the traditional classification of sleep states as uniquely mammalian or avian phenomena characterized by distinct rapid eye movement (REM) and NREM phases. Instead, it posits that a shared ancestral mechanism involving infraslow rhythms might have served as the primordial scaffold upon which diverse sleep architectures evolved.
Technological innovation was critical in enabling these findings. The researchers harnessed long-term electrophysiological recordings alongside physiological sensors capable of capturing ultra-slow oscillatory activity and subtle systemic changes. This multimodal approach allowed them to correlate brain rhythms with peripheral markers like heart rate variability and even skin pigmentation shifts in chameleons—an often overlooked but vital physiological indicator. Such precision and breadth in data acquisition represent a quantum leap in comparative neuroscience, permitting cross-species analysis of sleep phenomena at unprecedented resolution.
Moreover, the study raises intriguing questions about the functional relevance of these infraslow rhythms beyond their presence. For example, do these oscillations facilitate synaptic plasticity or memory consolidation across species? Are they involved in regulating neurochemical milieu during sleep, or perhaps in synchronizing neural ensembles across distributed brain areas? While initial evidence points to their integrative role coordinating physiological states, future research will be essential to delineate the mechanistic pathways by which these rhythms exert their influence on brain and behavior.
In ecological contexts, the conservation of this rhythm across species with differing sleep patterns and environments invites speculation about its adaptive significance. Lizards, which often exhibit polyphasic and light-sleep patterns, share this rhythm with mammals and pigeons despite profound differences in sleep duration, intensity, and architecture. This commonality suggests that infraslow brain rhythms might provide a baseline neurophysiological framework facilitating sleep’s essential restorative functions, making them resilient to divergent evolutionary pressures like predation risk or metabolic demands.
The study also offers potential translational relevance for human sleep disorders. Aberrations in slow oscillatory activity have been implicated in conditions such as insomnia and neurodegenerative diseases. Understanding the fundamental and conserved nature of infraslow rhythms could spur the development of novel diagnostic tools and therapeutics aimed at modulating these oscillations. The evolutionary approach emphasizes that targeting such deeply rooted physiological processes might yield robust benefits for sleep health spanning across lifespans and pathological states.
Beyond clinical implications, the research rekindles interest in comparative sleep research, emphasizing the value of reptiles and non-mammalian species for unlocking sleep’s mysteries. Historically understudied, these animals serve as living windows into the neurobiological past, offering simpler yet sophisticated models to decode the fundamental principles governing sleep. Their unique physiology, as evidenced by features like skin brightness modulation, provides alternative markers of brain activity that complement classical electrophysiological monitoring methods.
In sum, this landmark study pinpoints a crossroads where evolutionary biology, neuroscience, and physiology converge. By mapping infraslow sleep rhythms from lizards to humans, it paints a unifying portrait of sleep as a conserved neurobiological phenomenon. Its findings advocate for a holistic reinterpretation of sleep states not as mammalian or avian novelties but as extensions of ancestral rhythmic processes shared across amniotes. This paradigm shift opens new avenues for exploring how the brain harmonizes internal and external worlds during rest.
As the field advances, integrating molecular genetics, neuroimaging, and computational neuroscience with these cross-species electrophysiological insights will deepen our grasp of sleep’s origin, mechanisms, and functions. The evolutionary persistence of infraslow oscillations underlines sleep’s indispensability and complexity, beckoning further inquiry into how these timeless rhythms sculpt brain health and cognition. This work thus represents a monumental stride toward unlocking the evolutionary narrative encoded in the neural signature of sleep.
Subject of Research: Evolutionary conservation of infraslow brain rhythms associated with sleep across reptiles and mammals.
Article Title: Sleep-dependent infraslow rhythms are evolutionarily conserved across reptiles and mammals.
Article References:
Bergel, A., Schmidt, J.M., Barrillot, B. et al. Sleep-dependent infraslow rhythms are evolutionarily conserved across reptiles and mammals. Nat Neurosci (2025). https://doi.org/10.1038/s41593-025-02159-y
Image Credits: AI Generated
DOI: https://doi.org/10.1038/s41593-025-02159-y
Tags: advanced brain recording techniquesamniotes sleep patternsbrain rhythms in lizards and humanscommonality in sleep evolutioninfraslow brain rhythmsintegrated sleep signalsinterspecies sleep researchneural activity in reptiles and mammalsphysiological markers during sleepsleep architecture across speciessleep evolutionary originssynchronized sleep-related signals
