Skip to main content

<em>Science</em>: Reptiles Share Sleep Patterns with Mammals and Birds

A sleeping bearded dragon experiences some of the same sleep cycles as mammals and birds. | Stephan Junek, Max Planck Institute for Brain Research

The sleep patterns previously thought to be exclusive to mammals and birds are also found in reptiles, a new study reveals. The results, published in the 29 April issue of Science, shake up our understanding of the evolution of sleep.

Mammals and birds experience two types of sleep states. Slow-wave sleep (SWS) is characterized by electrical brain activity involving slow waves with a low frequency, a state that helps memory formation. Rapid eye movement sleep (REMS) is characterized by random movement of the eyes when SWS is not occurring, and is associated with vivid dreaming.

Until now, scientists believed that these sleep states evolved twice, once in mammals and again in birds, after these groups diverged from reptiles. However, Gilles Laurent of the Max Planck Institute for Brain Research and colleagues have uncovered SWS and REM sleep patterns in the Australian bearded dragon, Pogona vitticeps, suggesting that the sleep states may have evolved in a common ancestor of all three groups, more than 300 million years ago.

The discovery happened by chance.

Laurent said his team initially sought to study the reptilian cortex as a simpler, ancestral model of the cortex by examining the visual cortex of turtles. At the same time, they were monitoring the electrical brain activity in bearded dragons, because monitoring such activity is easier in this species than in the turtles.

Laurent said, "In the course of setting up an experiment we once, by curiosity, kept the recordings going overnight, while the [bearded dragon] slept. We discovered that brain activity oscillated regularly and in an uninterrupted fashion throughout the entire night."

Slow-wave sleep (SWS) and rapid eye movement sleep (REMS) may have evolved in the common aminote ancestor of birds, mammals (pink) and non-avian reptiles (blue). | Shein-Idelson et al., Science (2016)

The team decided to explore this finding further, implanting probes into the brains of five bearded dragons to measure their electrophysiological activity during sleep. Similar to humans and other mammals, the dragons demonstrated both SWS and eye movements during states of non-SWS, although the reptiles were found to cycle through these two states at much briefer duration and higher frequency. Whereas humans go through four or five 90-minute SWS-REMS cycles on average each night, the dragons experienced 350 80-second cycles, on average.

"Our results suggest that REM and SWS probably evolved at least as far back as with the common amniote ancestors of reptiles, birds and mammals, some 320 million years ago," Laurent said. "This is a long time ago, a time when the Earth's landmass was a single super continent."

The source of these cycles in reptiles also was surprising. In rodents, for example, SWS involves sharp-wave ripples in electrical brain activity that originate from the hippocampus. This phenomenon, where neurons "replay" memories from a wakeful state, is known to support memory formation. Yet, in the bearded dragons, the researchers found that these sharp-wave ripples originated from the brain's anterior dorsal ventricular ridge, rather than the hippocampus.

"We have no evidence for replay in dragons, but this is an obvious area to investigate now," said Laurent. "If brain activity during sleep serves memory-related functions in reptiles, [which) is only a hypothesis at this point, how did this principle evolve?"