Sometimes when birds and mammals are asleep and (presumably) dreaming, the eyes twitch rapidly and wildly. Now, researchers studying the brains of sleeping bearded dragons reveal that reptiles share similar sleep patterns with us and birds. In addition to rapid eye movement (REM) sleep, bearded dragons also exhibit a second sleep state called slow-wave sleep. The findings, published in Science this week, suggests that our brain sleep states evolved in a common ancestor some 300 million years ago. 

From worms to parakeets to cats, most animals sleep. But until now, researchers have only recorded the brain activity of sleeping birds and mammals. And the electrical patterns of our sleeping brains are divided into two main types: slow-wave sleep (SWS) and REM sleep. Periods of low frequency, high amplitude brain activity is followed by phases of awake-like brain activity and conspicuous rapid eye movement. 

To study the electrophysiological activity of sleeping reptiles, a Max Planck Institute for Brain Research team led by Gilles Laurent implanted a silicon probe and electrodes into the brains of five male Australian dragons (Pogona vitticeps). The lizards were kept on a 12-hour light followed by 12-hour dark cycle (with lights on at 7 a.m.) using custom-designed LED lights. A sensor for measuring heart rate was taped on the animals, and the team used infrared cameras to record and help quantify any eye movements in the night. 

The team found that the reptilian brains oscillated regularly between slow-wave and REM sleep patterns – like us. 

But compared to mammals, their sleep rhythm is both extremely regular and fast: The dragons cycled through the two states more often and for briefer durations. On average, we go through four or five 90-minute-long SW-REM sleep cycles, and REM sleep is much shorter than SW sleep. By contrast, the SW and REM sleep patterns observed in the lizards oscillated continuously for 6 to 10 hours with a period of about 80 seconds. They experienced 350 of these 80-second cycles on average, and their SW and REM sleep have roughly the same duration during each cycle. 

“Although similar to mammalian sleep, lizard SWS and REMS resemble a stripped-down (two-state) version of the richer mammalian repertoire (especially in comparison to sleep in large mammals),” the authors wrote.

The findings push the evolution of these sleep state dynamics to at least the emergence of amniotes – the large, terrestrial vertebrate lineage comprised of reptiles, birds, and mammals. Unlike fish or amphibians, amniotes had eggs that could survive outside of water, an ability that allowed us to colonize land. If SW and REM sleep occurs in bearded dragons, birds, and humans, then these date back at least as far as our last common ancestor – which lived around 320 million years ago. 

^{Image in the text: Australian dragon (Pogona vitticeps). Stephan Junek/Max Planck Institute for Brain Research}