Sleep is a universal biological state that allows all animals, from mammals to amphibians, fish and even insects, to restore their energy and consolidate knowledge that can contribute to their survival. Neuroscientists and zoologists have been investigating the biological underpinnings of sleep and its vital functions for centuries, more recently by measuring the brain activity of animals or people while they are asleep.
Recorded electrical signals that nerve cells produce while they are communicating with each other, also known as brain rhythms, have provided valuable insight into what happens during sleep. One of these rhythms, the so-called infraslow rhythm, had so far been primarily observed in mammals and was linked to a stage of sleep known as non-rapid eye movement (NREM) sleep.
Researchers at the Neuroscience Research Center of Lyon, PSL University, McGill University, University Jean-Monnet Saint-Etienne and other institutes recently recorded the brain activity of a wider range of animals and found that this ancient rhythm is common across several species, including reptiles, birds, rodents and humans.
Their most recent paper, published in Nature Neuroscience, reports the observation of the infraslow brain rhythm in seven different lizard species.
“I started this project in 2011 when I joined the sleep team at the Lyon Neuroscience Center, while now I moved to the functional and evolutionary ecology center of Montpellier,” Paul-Antoine Libourel, senior author of the paper, told Phys.org.
“My team at the time focused on understanding the function and mechanisms of REM sleep. When I arrived in the lab, my goal was to investigate the evolutionary origin of REM sleep by studying reptiles, which are ectothermic (cold-blooded animals) and share a common ancestor with mammals and birds—both homeothermic (warm-blooded animals) and both known to exhibit REM sleep.”
The main objective of the recent work by Libourel and his colleagues was to shed new light on the evolution of sleep states. Specifically, they wished to determine whether different states evolved separately following the appearance of warm-blooded animals, or whether some of them were already present in species that existed approximately 300 million years ago.
Recording brain rhythms in different animals
To study the brain rhythms of animals during sleep, the researchers implanted electrodes on or inside the brain of various specimens. These electrodes allowed them to detect electrical signals associated with neural activity, using a device that they designed specifically for this type of research.
“Because some lizard species could be small, we collaborated with the Lyon Institute of Nanotechnology to develop a miniature, low-power-consumption biologger,” said Libourel. “This logger is now commercialized by Manitty, a startup I co-founded and is used to record brain activity, physiology, and behavior in animals and humans in their natural environments (at home for humans).”
As part of an earlier study, Libourel and his colleagues used the device they created to record brain rhythms in penguins. Their new research, on the other hand, focused on different lizard species, namely the leopard gecko, tokay gecko, Sudan plated lizard, Argentine tegu, panther chameleon, Egyptian rock agama, and bearded dragon.
“In addition to brain activity, we also recorded physiological signals such as eye movements, heart rate, breathing rate, and muscle tone,” explained Libourel.
“Our electronics were designed to acquire all these signals simultaneously on a single board. Additionally, thanks to the support of Dr. Antoine Bergel, we had the opportunity to measure vascular activity using functional ultrasound imaging. We applied this technique in both mice and bearded dragons.”
A collective and ancient brain rhythm
The large dataset compiled by the researchers over the past decade or so led to an important and interesting discovery. Specifically, the team found that reptiles, mammals, and birds share a common brain rhythm, the so-called infraslow rhythm. This finding suggests the presence of an ancestral mechanism that dates back at least to 300 million years ago, when the earliest known ancestor of the species examined lived.
“This rhythm involves not only brain activity but also physiological processes and peripheral vascularization, indicating that it is a global, organism-wide rhythm,” said Libourel.
“The infraslow rhythm closely resembles a rhythm previously described in mammals during non-REM (NREM) sleep. In mammals, this rhythm has been proposed to play a role in brain ‘cleaning’ processes by facilitating the elimination of metabolic waste through cerebrospinal fluid flow. Additionally, because this rhythm is associated with fluctuations in vigilance, it may also represent an adaptive mechanism that allows periodic monitoring of the environment during sleep, potentially reducing the risk of predation.”
In the future, Libourel and his colleagues could test these two hypotheses in lizards. Their recent findings could also inspire other studies that investigate the brain rhythms of different animals during sleep, potentially leading to further interesting discoveries.
“A broader implication of our findings is that, if this rhythm reflects an NREM-related process in mammals, reptiles may not exhibit REM/NREM sleep as it is expressed in mammals,” said Libourel.
“In humans, REM sleep is strongly associated with dreaming. This does not imply that reptiles do not dream; rather, it suggests that their sleep-state organization differs from that of mammals, despite sharing some conserved processes such as the infraslow rhythm.”
The researchers plan to continue investigating brain rhythms and the evolution of sleep states. In their next studies, they could focus on other animal groups, such as amphibians and fish.
“The mechanisms underlying the infraslow rhythm also need to be deciphered in order to determine whether it serves the same functions as those proposed in mammals,” added Libourel.
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Publication details
Antoine Bergel et al, Sleep-dependent infraslow rhythms are evolutionarily conserved across reptiles and mammals, Nature Neuroscience (2025). DOI: 10.1038/s41593-025-02159-y.