Bearded dragons — they're just like us! They love their facial-hair trends. They struggle with social hierarchies (lizard cliques are the worst). And, they sleep in stages, a claim that would have earned a resounding "staaahhpp it" until, well, now. Researchers at the Max Planck Institute for Brain Research recently found that bearded dragons, aka pogona lizards, experience a simpler version of human "brain sleep." They published their findings in Science on April 29. As the New York Times reported, their discovery suggests that human sleep could have much deeper evolutionary roots than previously thought.
It's been a minute since the scientific community more or less accepted that all animals experience sleep or something like it. But, "brain sleep," characterized by cyclical stages of distinct electrical activity patterns, was thought to occur only in mammals and birds. Researchers believed that more primitive animals — e.g., fish, insects and various other invertebrates — lacked the neural infrastructure to support staged sleep.
Human shuteye consists four non-REM phases (two light, two deep) and REM sleep, when most vivid dreaming happens. Each five-stage cycle takes somewhere between 70 and 100 minutes. Over the course of a night, REM sleep takes up a greater proportion of each cycle.
By contrast, the lizard sleep-cycle takes all of 80 seconds. And the amount of REM holds steady throughout the amphibian's six-to-10-hour period of repose. During lizard slow-wave sleep, researchers observed bursts of electrical brain activity, which they compared to mammalian "sharp-wave ripples." But in humans, sharp-wave ripples, which emanate from the hippocampus, are involved in critical memory processes. The lizard brain, however, lacks anything equivalent to a hippocampus.
So, it's not a done-deal that the observed electrical bursts are sharp-wave ripples. "This rhythmic pattern reminds me very much of the up-and-down shifts that are the hallmarks of slow-wave sleep," Gyorgy Buzsaki, a neural sciences professor at NYU, said in the Times story.
The sharp-wave-ripple debate may not feel as game-changing outside academia. But, the findings sparks another, more accessible question: Do lizards dream?
Eh, well, it depends. If we think of dreaming as a subjective mental experience, then no. They don't meet the consciousness requirement. But, if we think of dreams as "bits of neuronal playback" in various brain regions, explained study author Gilles Laurent to the Times, then sure thing.
But, the big takeaway concerns the evolutionary roots of sleep. If primitive animals enjoy staged-shuteye, then it's likely that sleep evolved a very long time ago in a common ancestor.
Per the Times:
Daniel Margoliash, a professor of organismal biology and anatomy at the University of Chicago, said the study provided “extremely strong evidence that the patterns of structure of sleep that we’ve seen in a broad range of species is reflective of something that evolved very early in vertebrate evolution and is shared across many — perhaps all — vertebrates.”
Regardless, we owe bearded dragons and their scaly kinfolk an apology. They, too, have been to REM and back again.