Just like humans, lizards experience two different sleep states, suggesting the cold-blooded creatures dream.

The brains of all animals perform important functions during sleep -- memories get processed and organized, the metabolic trash gets taken out, neuronal energy reserves get replenished. But until now, scientists thought only land mammals and some birds experienced two different sleep states, slow-wave sleep and REM.

Slow-wave sleep, or deep sleep, is important for memory formation and general recovery. Scientists think all animals experience some form of deep sleep. REM sleep, or paradoxical sleep, is less common. Scientists associate REM sleep with dreaming.

In the new study, scientists at the Lyon Neuroscience Research Center in France replicated a 2016 study that found bearded dragons experience two distinct sleep states. The replicated experiments confirmed the findings.

Scientists also observed the sleep patterns of another lizard species, the Argentine tegu, or Salvator merianae. Like bearded dragons, the Argentine tegu alternatives between two different sleep states during shuteye.

The newest study, published this week in the journal PLOS Biology, offered new insights into differences between reptilian sleep states and those experienced by mammals.

REM sleep experienced by humans and other mammals features cerebral and ocular activity similar to the patterns measured while awake. Scientists found both lizard species exhibited slower eye movements during REM sleep. They also found tegu lizards exhibit cerebral activity distinct from patterns measured during waking hours.

The findings suggests similar sleep states can potentially support different functions in closely related species.

"Our results suggest a common origin of two sleep states in amniotes," researchers wrote in their paper. "Yet, they also highlight a diversity of sleep phenotypes across lizards, demonstrating that the evolution of sleep states is more complex than previously thought."

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