A team from the Centre National de la Recherche Scientifique (CNRS) in France identified a ultra-slow biological pattern present during sleep in reptiles and birds, identical to that which characterizes deep slow-wave sleep in mammals.
The discovery, published in Nature Neuroscience, suggests that this mechanism originated in a common ancestor prior to the great evolutionary divergence between synapsids, sauropsids, and archosaurs, more than 300 million years ago.
Broad and multimodal methodology
The research was distinguished by a comprehensive approach: scientists did not limit themselves to the electroencephalogram but conducted a simultaneous monitoring of brain, cardiac, vascular, respiratory, muscular, and ocular activity during complete sleep cycles.
Ten vertebrate species were studied: seven reptiles (geckos, bearded dragons, turtles), one bird (parakeets), and two reference mammals. A key technique was functional ultrasound imaging, which allowed visualization of vascular dynamics and blood flow associated with sleep.
Compelling results
The analysis of large volumes of data showed that in reptiles and birds there is a slow and regular oscillation, synchronized across different organic systems.
This rhythm coincides with deep sleep in mammals, characterized by high amplitude and low frequency brain waves, a crucial stage for memory consolidation and metabolic waste elimination through the glymphatic system.
Evolutionary implications
The conservation of this pattern over hundreds of millions of years indicates that it is not a recent adaptation but a central and indispensable mechanism for the function of sleep in amniote vertebrates. Its persistence suggests a critical role in survival, maintained by selective pressure through generations and environmental changes.
The researchers propose that, in addition to possible neural cleaning functions, this rhythm could be an adaptive strategy to minimize energy consumption, reduce detectability to predators, and facilitate a rapid transition to wakefulness in case of threat.
Debate on paradoxical sleep
The study also sheds light on the debate about the existence of an equivalent to REM sleep (rapid eye movements) in reptiles. The exhaustive monitoring found no evidence of a state that combined all the characteristics of mammalian paradoxical sleep.
Instead, the research emphasizes that the basic architecture of sleep in reptiles and birds is based on this ancestral slow rhythm, placing the origin of REM sleep in a later evolutionary stage, probably specific to mammals or warm-blooded amniotes.
The finding reveals a hard core of sleep shared by lizards, birds, and humans, a slow and persistent rhythm that connects the present neurological state with a distant past. This oscillation stands as the physical signature of an ancestral brain function, an internal clock marking the beat of an essential restorative process for life, whose mystery has yet to be fully unveiled.
