Snoring Lizards Reveal That Sleep Predates Dinosaurs

Sleep patterns like REM and SWS evolved in a common animal ancestor more than 300 million years ago.

Owners of pet lizards for years have anecdotally reported that their reptilian friends snore, appear to dream, and seem to enjoy snoozing just as we do, and now new research finds that reptiles do indeed experience the same sleep states of mammals and birds.

Before, it was thought that only birds, humans and other mammals went through rapid eye movement sleep (REM) and slow-wave sleep (SWS) as they snoozed, but the new findings, published in the journal Science, suggest that these sleep patterns evolved in the common ancestor of all such animals more than 300 million years ago.

By extension, it is then likely that today's reptiles dream, and that many long-extinct animals such as dinosaurs did so as well.

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"Dreaming, like sleep, consciousness, language, pain etc. are all concepts or phenomena that were first experienced and described in a self-referential manner by humans, assigned a word and implicitly considered as uniquely human," senior author Gilles Laurent, director of the Max Planck Institute for Brain Research, told Discovery News.

"But if you think as a biologist about these phenomena, accept that most of them probably did not drop from the sky onto the first human being, but rather result from some slow evolutionary process, then we can start thinking about dreams as patterns of neuronal activity in the brain during sleep that are at least partly built up from past experience," he added. "If you are ready to accept that bits of neuronal playback in certain brain areas during sleep can be called dreams, then I'll bet that lizards dream."

Scientists for decades tried to detect sleep patterns in reptiles by placing electrodes on the surface of the animals' brains, similar to doing an EEG. Laurent and his team took this a step further, by implanting probes right into the brains of five bearded dragons (lizards). As a result, the researchers were better able to record the sleep patterns and to match them to eye movements.

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Humans go through four or five 60 to 90 minute SWS-REM cycles on average every night, but the lizards experienced about 350 80-second cycles. Rest is unaffected, Laurent said, as the two sleep cycles run together, just as they do in humans. The individual is basically unaware that all of this brain wave change is happening.

"Among mammals, there tends to be some relationship between animal size and length of sleep cycle," he said. "It is not absolute, but is statistically true. For example, the human sleep cycle is about 60–90 minutes, while it is 30 minutes for cats and 15 minutes for rats."

Even insects and arachnids like spiders sleep, but it is unclear if they experience REM and SWS sleep patterns. Some animals rest, but do not technically sleep.

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Laurent explained, "Multicellular animals without brains, such as sponges and jellyfish...most likely do not express 'brain sleep' both by definition since they lack a brain and because REM and SWS require certain neural circuits that, as far as we know, they lack."

Even for these creatures, prior amounts of rest, temperature shifts and other environmental factors seem to affect rest, which in multicellular animals without brains probably is expressed through their circadian rhythms, otherwise known as their internal "body clocks."

Snoring Lizards Reveal Sleep Predates Dinosaurs: Page 2

An Australian central bearded dragon get REM sleep. | Arthur Georges

As for now-extinct dinosaurs, Laurent said that "it seems likely they expressed REM and SWS." They therefore probably dreamed and consolidated their memories as they slept in a way that was at least somewhat similar to how we and many other animals do so today.

Thanos Siapas, a professor of computation and neural systems at Caltech, told Discovery News that the new study "is a truly groundbreaking paper that sheds new light into how the architecture of sleep evolved. The data and analysis are very compelling and suggest an ancient origin of the two alternating stages of sleep."

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Siapas and the researchers were surprised that the sleep patterns detected in the lizards originated from different parts of the brain than expected, based on what happens in mammals and birds. Siapas hopes that future research will explore why this occurs, clarifying the functions of those brain regions.

Then there is what Laurent called "the $64 million dollar question": Why and how did sleep evolved in the first place?

"We do not know now, and this is a fascinating question," he said. "Did it evolve as something completely different, and evolve into what we know today? It is too early to say."

An iguana is shown taking a snooze.

March 13 has been deemed World Sleep Day by the World Association of Sleep Medicine, which hopes to call attention to problems that arise from sleep disorders. Presumably, the sleep researchers were thinking about humans when they declared the day, but it made us think of animals, too. After all, many critters don't need any reminders about the importance of sleep. They're already so on top of the job that some animals sleep


of the day (and we're looking at you on that, koalas!). Let's tip-toe past some of nature's big sleepers as they snooze away or get ready for a nap. We start right in the average pet owner's home, with cats and their sometime chums, dogs. We're told to let sleeping dogs lie, and this one's feline friend seems to be heeding the advice.

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Koalas are super-sleepers, racking up about 20 hours per day toiling away at their slumber. If sleep were a superpower, koalas would have their own Hollywood superhero tentpole franchise built on the premise that they fight crime, but only between naps.

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Sloths in captivity have been known to sleep some 15 hours per day, but in the wild they don't clock as much shut-eye. Fairly or not, though, they're right up there with koalas in being associated with sleep.

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In the jungle, the mighty jungle, the lion sleeps tonight. Okay, not quite -- lions are largely nocturnal and do most of their sleeping during the day, as we can see here. The king of the jungle sleeps a bit more than half its day, for around 13 hours.

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Tigers are as sleepy as lions, often a bit more so, lounging for about 16 hours each day. This Bengal cub seems to have gotten the hang of it.

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This yawning bat is ready for some hanging-upside-down time.

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How is a lemur supposed to get any sleep with all this racket? This one tries the shield-your-eyes trick.

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Horses are known to take a load off, perhaps even bedding down for a snooze amid some daisies.

A sleepy red panda dreams of bamboo shoots.

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And, lastly but not leastly, squirrels sleep for about 15 hours per day, giving lions and tigers a run for their money.

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