Have Evolutionists Found the Root of REM Sleep?

News to Know


Do bearded dragons aid the quest for the evolutionary origin of REM sleep and dreams?

Dragons sleep, and they probably even dream. Though this could likely be said of Smaug the Magnificent sleeping for centuries atop his golden hoard in Tolkien’s Lonely Mountain of Hobbit fame, scientists have now shown that the Australian bearded dragon, popular as a pet worldwide, has sleep cycles analogous to those in cats, birds, and everyone reading this article. And since dreams happen during our REM sleep, it is not unreasonable to imagine that lizards truly embark on their own dream quests while their eyes twitch.

To Sleep, Perchance to Dream

All sorts of animals sleep, even the lowly roundworm. And sleep is important. A fruit fly has a tough time learning and remembering simple lessons—like how to avoid bitter smells and nasty shocks—when deprived of sleep.1 But not all animals have sleep like ours, characterized by alternating cycles of distinctive electrical activity in the brain. Until now, it was thought that only mammals and birds experienced REM and non-REM sleep. The discovery of these alternating sleep patterns in lizards suggests, according to the authors of “Slow Waves, Sharp Waves, Ripples, and REM in Sleeping Dragons,”2 that these complex sleep cycles evolved at least 320 million years ago in animals distantly ancestral to all mammals, reptiles, and birds.

Santa Lizard

This is Tugboat, a bearded dragon living in the United States. Others of her species have revealed to German scientists that bearded dragons experience cycles of rapid eye movement (REM) and slow wave sleep comparable to the patterns of electrical brain activity seen in mice and, of course, in us. We dream during REM sleep. Perhaps at night Tugboat has visions of sugary bugs dancing in her head! Photograph courtesy of Katie McDuffee.

Human beings spend a lot of time sleeping, and disturbance of our normal sleep cycles is associated with many problems from poor productivity to psychological issues. Each phase of sleep is characterized by particular electrical patterns produced by the neurons in the brain. REM sleep is associated with high frequency electrical activity. Non-REM sleep is also called slow wave sleep because it is associated with slower, low frequency electrical activity.

Humans alternate between REM and non-REM (slow wave) sleep in a cycle that repeats roughly every 60–90 minutes about 5 times a night. Birds—who only let half their brain sleep soundly at a time3alternate brief 9-second REM phases with 2.5-minute non-REM phases.4 Dogs typically experience shorter sleep cycles than ours but much longer than those of birds, with cycles lasting around 15-minutes about 20 times a night.5 Dreams are associated with REM sleep. Like people, dogs awakened in the midst of REM sleep are prone to be confused and grouchy, perhaps the source of the popular and prudent advice to let sleeping dogs lie.

Mice, like humans, experience rapid-eye movement (REM) and non-REM (slow-wave) sleep, though their sleep cycles whiz by at warp speed compared to our leisurely 60–90 minute cycles. These two types of sleep are identified by different patterns of electrical activity in the brain and—you guessed it—the presence or absence of rapid eye movements. Mammals—like people, dogs, and mice—also tend to lose their muscle tone during REM sleep, relaxing completely. Fortunately for our feathered friends perching in trees, birds do not.6

Though we know that both REM and non-REM sleep are important, their respective roles in memory formation are still being worked out. Mice specifically deprived of their REM cycles while they sleep, scientists recently confirmed, fail to remember familiar objects and even the places they’ve been conditioned to fear.7 This affirms what teachers everywhere tell their students at exam time: Forget all-nighters—a good night’s sleep is important if you want to remember what you’ve studied. Whether or not REM sleep helps dragons recall the best places to find their favorite bugs and veggies—or how much treasure is in their hoard—has also not yet been worked out. However, now that scientists know bearded dragons experience REM sleep comparable to that in rodents, time and a few more studies should soon tell us if REM-deprivation dumbs down dragons the way it does mice and people.

The Dragon Sleeps Tonight

The bearded dragon species Pogona vitticeps is popular as a pet, making it readily available for research at Frankfurt’s Max Planck Institute for Brain Research. But institute director Gilles Laurent and colleagues didn’t start out trying to discover the evolutionary origin of REM sleep, or even whether dragons dreamed. They originally implanted electrodes in their dragons’ brains to analyze the electrical activity associated with tracking tasty treats. However, the scientific method has a way of rewarding the observant, so the serendipitous discovery of cyclic electrical activity while the dragons snoozed sent the team of neuroscientists on a different dragon quest.

Sleeping Dragon’s Brain Activity

The sleeping dragon’s brain activity reveals a repetitive pattern reminiscent of what we see in humans, played at high speed. In this screen shot from the study in Science, the shaded area in A is expanded in Ab, and shaded areas in Ab are further expanded in Ac and Ad, in order to show that the lizard brain waves have periods of short, rapid, REM-like activity alternating with taller, slower waves like those seen in non-REM sleep. Image from Mark Shein-Idelson et al., “Slow Waves, Sharp Waves, Ripples, and REM in Sleeping Dragons,” Science 352, no. 6285 (2016): 590–595, doi:10.1126/science.aaf3621.

When dragons are asleep is easy to discern. Their heart rate slows as they close their eyes. They relax in a horizontal posture with their heads resting on the floor.8 The frequency and duration of electrical wave patterns vary a lot between species, be they mice, men, birds, or lizards. But Laurent explains, “The more we looked, the more it appeared as though we were looking at bona fide REM sleep.”9

While dragons sleep, there is clearly an alternating pattern of low (about 4Hz) and high (about 20Hz) frequency neuronal activity in their brains. These phases alternate about every 40 seconds. The resulting 60–90 second cycle repeats 300–350 times each night. The phases of rapid electrical activity are, like in mammals and birds, associated with observable rapid eye movements. Furthermore, following each round of slow waves there is a burst of sharp waves of high voltage followed by electrical ripples. This pattern, also seen in mammals, is thought to represent a time when the brain is rapidly replaying events to cement them in memory. Finally, like the electrical activity produced during sleep in other animals, the electrical activity found in dragons originates in an identifiable part of the brain. However, the brain region responsible for making sure dragons get the sort of sleep they need (the dorsal ventricular ridge) in no way resembles the brain region that generates the rhythm of sleep in mammals (the hippocampus).

Future behavioral studies will likely reveal whether dragons, like mice and people, need their sleep cycles to remember their day’s adventures. Those sorts of things can be tested in the laboratory. But do dragons dream? They may well, but they cannot tell us any more than our dogs can explain their nocturnal adventures to us. “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,” Laurent explains.10 Some things are just beyond the ability of scientists to observe. However, Laurent says, “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.”11 Of course what dragons dream about will have to remain a matter of speculation.

The Roots of REM

Scientific observation can confirm the importance of sleep in each species. And scientists can identify through observation where in the brain of an animal or person the electrical patterns of sleep cycles originate. But can science reveal the evolutionary origin of sleep cycles? Evolutionist Laurent thinks so. He says, “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.”12

Until now, most evolutionists have thought that complex sleep—a shared characteristic of mammals and birds—evolved in association with the evolution of warm bloodedness. After all, both mammals and birds are warm blooded. The idea is that lots of muscle activity—which occurs during non-REM sleep—was presumably needed to maintain the body’s core temperature in primitive mammalian ancestors. Then as the ability to maintain body temperature became more fully developed over evolutionary time, this became less necessary, allowing the complete relaxation of the muscles seen in REM sleep to become the nighttime norm. Of course scientists cannot measure the brain wave activity of extinct animals, so how they slept is as much a matter of speculation as their supposed position in the evolutionary history of mammals. However, since the sleep cycle is important to us, it was presumably important to these hypothetical ancestors and offered them an evolutionary advantage. Our complex sleep cycle has generally been seen as an evolutionary leftover, but a valuable leftover since the sleep cycle is important for long-term memory formation, for proper growth and development, and for health itself.

Did cyclic electrical brain patterns associated with sleep have to evolve more than once?

But did cyclic electrical brain patterns associated with sleep have to evolve more than once? Laurent believes his study shows that evolution only had to come up with the REM/non-REM pattern once, in a distant ancestor shared by all reptiles, birds, and mammals. “Positing convergent evolution (two or three times in amniote evolution) of a complex phenomenon such as sleep brain dynamics is a lot less plausible than imagining a common origin,” Laurent explains.13

Evolutionists believe that the common ancestor of amniotes—animals whose fertilized eggs could be laid on land or could develop inside the mother’s body—evolved 320 million years ago and soon diverged into a mammalian branch and a reptile-and-bird branch. The lineage containing bearded dragons supposedly branched off from other reptiles about 250 million years ago, long before the evolution of dinosaurs and hence (in the evolutionary view) birds. Thus, Laurent concludes, “Given the early branching out of the reptiles, additional evidence from several of reptilian branches such as turtles, lizards, or crocodiles will only increase the probability that we are looking at a common origin. The evidence, thus far, points to an origin of REM and slow-wave sleep at least as far back as the common ancestor of reptiles, birds and mammals, which lived about 320 million years ago.”14

In truth, this study of sleeping dragons reveals nothing about the evolutionary origin of sleep. It does show that the common design for sleep seen in terrestrial mammals and birds is also present in at least one sort of reptile.15 But Laurent’s conclusions about the evolution of sleep are based not on observations but only on his own evolutionary presuppositions, his faith in the evolutionary tree of life, which defies observational science by supposing that one kind of animal can actually evolve into another kind of animal. No mechanism for this sort of change is found in experimental biology.

The Gift of Sleep

Sleep is a marvelous design that came down from our shared Creator to us and all creatures needing rest. Sleep did not just drop from the sky onto the first human being, but neither did it result from some slow evolutionary process. To assume that those are the only possible explanations for sleep’s deep history ignores the ultimate eyewitness to history. The evolutionary worldview sees common designs as evidence of either shared evolutionary ancestry or convergent evolution, failing to recognize the biblical alternative: that common designs are evidence of the common Designer shared by all living things.

Common designs are evidence of the common Designer shared by all living things.

God—our Creator and the only eyewitness to all of history—has given us the true history of our origins in His Word, the Bible. All that God created was good (Genesis 1:31) before the curse of sin entered this world at the hands of rebellious man. Evening and morning began in the beginning, according to chapter one of Genesis, suggesting that the need for rest was part of that good world. Science reveals the common need for rest, the ubiquitous nature of sleep in all its varieties, and the remarkable similarities of sleep across many species. These observable facts about sleep illustrate the thread of a common design by our common Designer. Sleep—surely one of God’s many good gifts (James 1:17)—illustrates the hand of our wise Creator, who was able to masterfully work out the differences and the similarities and the specifics of sleep suited to the needs of human beings and to every earthly creature needing rest.

We cannot long be at our best without sufficient sleep, the sort designed for us by God. Likewise, we humans, being created in the image of God, cannot be at our best without another kind of rest. Just as our loving heavenly Father has made the rest of sleep freely available for our bodies, He has also made a kind of spiritual rest, peace between Himself and rebellious human beings, available to us. This rest for our souls, spoken of by Jesus Christ, is His gift available to those who obey His urgent command, “Come to Me” (Matthew 11:28–29). By His sacrificial death on the Cross to pay the price for our sin, Jesus Christ the Son of God has purchased salvation for all who repent and trust in Him. We hope everyone who reads this will not only marvel at God’s gift of sleep for His many creatures, but will also enjoy the spiritual rest only available to those who put their faith and trust in Jesus Christ.

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Answers in Depth

2016 Volume 11


  1. Fruit flies’ sleep, like sleep in other animals, is characterized by a time of inactivity in which greater-than-normal stimulation is required to arouse them. It turns out they need a good night’s sleep just as we do in order to perform at their best.

    Fruit flies are trained to avoid things they don’t like—nasty smells or shocks—even if it requires going against their natural instincts, such as a preference for light over darkness. And their memory of other lessons, like how to recognize a female fruit fly not interested in courtship, is also readily observable. The fruit fly’s ability to learn and remember such lessons is impaired by sleep deprivation.

    The importance of sleep in fruit flies has been shown by numerous studies, and scientists have therefore, for some time, been investigating the effect of sleep deprivation on the structures and neurotransmitters in the fruit fly brain. (To learn more, see Stephane Dissel, Krishna Melnattur, and Paul J. Shaw, “Sleep, Performance, and Memory in Flies,” Current Sleep Medicine Reports 1 (2015): 47–54, doi:10.1007/s40675-014-0006-4; and Laurent Seugnet et al., “D1 Receptor Activation in the Mushroom Bodies Rescues Sleep Loss Induced Learning Impairments in Drosophila,” Current Biology 18, no. 15 (August 2008): 1110–1117, doi:10.1016/j.cub.2008.07.028.)

  2. Mark Shein-Idelson et al., “Slow Waves, Sharp Waves, Ripples, and REM in Sleeping Dragons,” Science 352, no. 6285 (2016): 590–595, doi:10.1126/science.aaf3621.
  3. Birds spend most of their sleep time in slow-wave sleep, and they can regulate which part of the brain sleeps by keeping one eye open. This seems to be a general feature of birds. Birds spend only about a tenth of their sleep time in REM sleep, which cannot be confined to a single hemisphere. To learn more see Niels Rattenborg, Charles J. Amlaner, and Steven L. Lima, “Unilateral Eye Closure and Interhemispheric EEG Asymmetry During Sleep in the Pigeon (Columba livia),” Brain, Behavior and Evolution 58 (2001): 323–332, doi:10.1159/000057573.
  4. This estimate comes from “How Do Birds Sleep?,” at Audubon California, http://ca.audubon.org/news/how-do-birds-sleep. Estimates vary, however, given the difficulty of measuring brain wave activity in many species of birds. Bird sleep cycles are quite short, thought to last only about 1–4 minutes with only about 10% of the cycle consisting of REM sleep. From Albrecht Vorster and Jan Born, “Sleep and Memory in Mammals, Birds and Invertebrates,” Neuroscience and Biobehavioral Reviews 50 (March 2015): 108, doi:10.1016/j.neubiorev.2014.09.020.
  5. Estimates vary and may also differ by dog variety. This estimate comes from “Canine Sleep Cycles,” Doggies.com, http://www.doggies.com/articles/243-canine-sleep-cycles.html.
  6. According to the Cornell Lab of Ornithology, when a perching bird sleeps, its weight causes its leg muscles to tighten the tendons in its feet, gripping the perch securely. See “Do Birds Sleep?,” BirdSleuth K–12, http://www.birdsleuth.org/birds-sleep/.
  7. Learn more in “REM Sleep is a Keystone of Memory Formation in Mice,” Neuroscience News, May 13, 2016, http://neurosciencenews.com/memory-formation-rem-sleep-4236/, and Laura Sanders, “Brain Waves in REM Sleep Help Store Memories,” Science News, May 12, 2016, https://www.sciencenews.org/article/brain-waves-rem-sleep-help-store-memories. Also see Albrecht Vorster and Jan Born, “Sleep and Memory in Mammals, Birds and Invertebrates,” Neuroscience and Biobehavioral Reviews 50 (March 2015):103–119, doi:10.1016/j.neubiorev.2014.09.020.
  8. Mark Shein-Idelson et al., “Slow Waves, Sharp Waves, Ripples, and REM in Sleeping Dragons.”
  9. Emily Underwood, “Do Sleeping Dragons Dream?,” Science, April 28, 2016, http://www.sciencemag.org/news/2016/04/do-sleeping-dragons-dream.
  10. Jennifer Viegas, “Snoring Lizards Reveal Sleep Predates Dinosaurs,” Discovery News, April 28, 2016, http://news.discovery.com/animals/dinosaurs/snoring-lizards-reveal-sleep-predates-dinosaurs-160428.htm.
  11. Ibid.
  12. Ibid.
  13. Max-Planck-Gesellschaft,“Do Bearded Dragons Dream? Reptiles Share Sleep Patterns with Mammals and Birds,” Science Daily, April 28, 2016, https://www.sciencedaily.com/releases/2016/04/160428152134.htm.
  14. Ibid.
  15. It may be that these patterns were easier to detect and define in this study than in other sleep studies of reptiles because the electrodes were implanted in the brain rather than relying on external detection. We will have to await further studies to learn how widespread REM is in reptiles.


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