A new study of reptilian slumber raises questions about how our own pattern of sleep evolved, while suggesting that at least some lizards might enter dream states.
Mammals, including humans, and birds sleep in two distinct periods: slow wave and rapid eye movement (REM). A 2016 study showed that bearded dragons (Pogona vitticeps), a type of lizard, also vacillate between similar sleep phases.
Now, researchers led by Paul-Antoine Libourel of the Universite Claude Bernard Lyon 1, in France, have replicated that finding and demonstrated that that another lizard, the tegu (Salvator merianae), has a different biphasic sleep pattern.
While the bearded dragon could be said to have a slow wave and REM cycles, the tegu, which is common to Central and South America, “displays a type of oscillation never previously reported”, the researchers reveal in the journal PLOS Biology.
The study looked at brainwave oscillations, muscle tone, and eye movements of the reptiles.
The authors suggest that studying how lizards sleep will help determine how sleep states developed in humans, because all vertebrates share a common ancestor. The results suggest the process was not straightforward.
The different brain wave oscillations found in the two lizards, Libourel and colleagues write, suggest “that the phenotype of sleep states and possibly their role can differ even between closely related species”.
Nevertheless, they add, the existence of two types of sleep in both the dragon and the tegu suggests an origin in the common ancestor of reptiles, birds and mammals.
“Yet, they also highlight a diversity of sleep phenotypes across lizards, demonstrating that the evolution of sleep states is more complex than previously thought,” they conclude.
For something so ubiquitous, we know very little about how sleep developed, or why.
“If we didn’t need eight hours of sleep and could survive on six, Mother Nature would have done away with 25% of our sleep time millions of years ago”, neuroscientist Matthew Walker told US radio service NPR’s Hidden Brain program in 2017.
“Because when you think about it, sleep is an idiotic thing to do. If sleep does not provide a remarkable set of benefits, then it’s the biggest mistake the evolutionary process has ever made.”
For whatever reason, sleep is necessary. For humans, chronic sleep deprivation has been linked to brain disorders, and going too many hours without sleep is considered so dangerous that the Guinness Book of World Records no longer accepts submissions in the category
Whatever sleep offers, we all need it. It is an amazingly widespread phenomenon throughout the animal kingdom. One 2017 study found that even jellyfish sleep. So far, though, no word on whether they dream.
Samantha Page is a science journalist based in Spain.
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