Scientists have just revealed something pretty wild: a very slow brain rhythm, called an infraslow rhythm, shows up not just in humans and other mammals, but also in birds and a bunch of lizard species — including geckos. And this pattern isn’t a recent trick that evolved in warm-blooded animals. It’s ancient — stretching back about 300 million years to a common ancestor way before mammals and birds split off from reptiles.
To figure this out, researchers built tiny brain-recording devices (biologgers) that can sit on or inside animals and capture brain waves, heart rate, breathing, muscle tone, eye movements, and even blood flow at the same time. Using these on seven different lizard species, and comparing to birds, rodents, and humans, they found a very slow rhythm in the brain that cycles over tens of seconds. That same rhythm in mammals shows up during non-REM sleep — a phase tied to things like restoration and brain maintenance.
Here’s the mind-bender: because lizards, birds, and mammals all show the same slow rhythm, it suggests this feature of brain activity was present in a very early common ancestor, rather than evolving independently later on. This rhythm isn’t just brain electrical activity — it’s linked to whole-body signals like heartbeat and breathing — so it seems to be a fundamental organism-wide pattern during rest.
Scientists still don’t know what this rhythm does in reptiles — whether it helps with something like metabolic cleanup during sleep (as suspected in mammals) or something like periodic environmental monitoring — but its deep evolutionary roots are now clear. It also challenges how we define sleep stages across species: reptiles don’t seem to have the same neat REM vs non-REM pattern we humans do, even though they share this foundational rhythm.