Reptiles experience the same stages of sleep as humans do, according to a new study, which suggests dinosaurs may have slept like humans as well.
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Laurent and hs colleagues hooked their bearded dragons to an electroencephalogram (EEG) and found that they have the same sleep cycles as their avain and mammalian relatives – at some point, their brains were active as if awake, their blood pressure rose, and their eyes movied rapidly under their lids.
The discovery suggests the sleep stages evolved a lot earlier than previously thought, from a common lizard-like ancestor of mammals, birds and reptiles between 300 million and 320 million years ago.
The fact that reptiles, birds and mammals share similar sleep patterns suggests these evolved in a common ancestor some 300 to 320 million years ago.
A recent study published on Thursday, which involved 5 Australian bearded dragons, determined that this lizards may sleep and dream as birds, people and other mammals do. More specifically, it had a phase characterized by low frequency/high amplitude average brain activity and rare and bursty neuronal firing. Though the original intent of the research was not to discover if lizards dreamed, now that the discovery has been made that they do dream, Dr. Laurent and his team of scientists want to find out how long ago the three stages of sleep originated. When asked on what reptiles can possibly dream about, director of the Max Planck Institute for Brain Research in Germany and neuroscientist Gilles Laurent used a loose definition of dreaming to answer the question.
The researchers also noticed that the lizards’ sleep cycles had roughly equal parts REM and slow-wave sleep, whereas REM sleep in the human cycle tends to be shorter than slow-wave sleep and irregular in birds. So, what do lizards dream about, anyway? These electrical features of brain sleep, whose functions are not well understood, have so far been described only in mammals and birds, but not in reptiles, amphibians or fish.The researchers implanted a type of silicon probe in the forebrain regions of five Australian bearded dragons, or Pogona vitticeps, that tracked brain activity as the reptiles slept. “A REM phase with broadband activity and high-frequency activity, and slow-wave sleep where there is no eye movement”.
The dragon is sleeping, scientists say.
They also report interesting differences: for example, lizard sleep rhythm is extremely regular and fast: the lizard’s sleep cycle is about 80 seconds long at 27oC, vs. 30 minutes in cat or 60 to 90 minutes in humans. But the finding, they added, is bound to generate more controversy about whether the resting state of primitive animals is really the same as sleep, and whether the brain activity seen in a lizard can be compared to that in mammals. At that time the earth’s continents formed a single landmass.
“We had a characterization that was starting to become more and more like what we observe in mammalian sleep”, Laurent said.
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Lizard sleep may be similar to the sleep patterns of our ancestors, said Dr Laurent. “But if there is evidence that [non-avian] reptiles also have REM sleep and slow-wave sleep, then the most plausible explanation is that it actually existed in their common ancestor”.
