REM sleep has ancient evolutionary origins
Almost all animal species sleep. When mammals and birds sleep, their brains stay active and researchers can identify various phases of sleep by measuring the brain’s electrical activity.
For the first time, neuroscientists have recorded the brain activity of a sleeping reptile, the Australian bearded dragon. Their results, described in an article published in the journal Science, reveal the typical features of slow wave and rapid eye movement (REM) sleep that were previously thought to be restricted to mammals and birds.
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At first, the researchers simply noticed that brain activity recorded from resting bearded dragons oscillated regularly between two states. By classifying the brain activity patterns based on statistical, dynamical, and anatomical features, and then correlating them with observed behaviors such as the presence or absence of rapid eye movements, the researchers were confident that the two states they were seeing were in fact slow wave and REM sleep.
Birds and mammals are both descended from reptiles. Together, birds, mammals, and reptiles make up a group called amniotes, which refers to vertebrates that live on land.
The fact that a reptile was found to share sleep patterns with birds and mammals suggests that these patterns were present in the common ancestor to all three groups. Brain sleep therefore probably dates back to the first colonization of land by vertebrates — upwards of 320 million years ago.
These findings indicate that the brain circuits responsible for slow-wave and REM sleep are more ancient than was previously thought.
REM sleep is closely associated with dreaming in mammals, which begs the question: Do these sleeping dragons dream?
Scientists still don’t know the answer. But that shouldn’t stop us from pondering the next logical question: If they do dream, what do they dream about?
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