Dragons sleep like mammals and birds
Finding slow-wave and REM sleep in reptiles could change our understanding of how sleep evolved
Lizards might snooze like people do.
Sleeping lizards seem to share distinctive brainwave patterns with sleeping birds and mammals. If true, these new findings suggest that sleep patterns in people evolved in a common ancestor of birds, mammals and reptiles. That would mean these patterns already existed some 300 million years ago. Researchers reported the finding April 29 in Science.
During sleep, the brains of mammals and birds alternate between two states. One is deep, slow-wave sleep. During this stage, recordings of the brain’s electrical activity show occasional bursts of big, slow waves. The other state is called rapid eye movement, or REM, sleep. Here, brain waves appear small and fast, like those of an awake brain. An animal’s eyes usually twitch quickly during this phase of sleep.
“The prevailing view has been that REM and slow-wave sleep are limited to mammals and birds,” says study coauthor Gilles Laurent. He’s a neuroscientist at the Max Planck Institute for Brain Research in Frankfurt, Germany. Many researchers think that birds and mammals separately evolved this similar sleep pattern, Laurent says. So he and his colleagues were surprised to find the familiar patterns in the dozing lizards they studied, called Australian dragons (Pogona vitticeps).
“We saw this two-state sleep in the lizard, which we definitely did not expect,” says study coauthor Mark Shein-Idelson. He’s another neuroscientist at the Max Planck Institute for Brain Research.
The researchers implanted probes in the dragons to study electrical activity in their brains. This turned up a pattern of two alternating states as the dragons slept. One state looked looked like slow-wave sleep. The other looked like REM sleep.
Their sleep cycles were simple and speedy, though. Each cycle lasted around 80 seconds, on average. Indeed, the lizards cycled through these states hundreds of times each night. In contrast, mammals’ cycles last much longer — roughly 30 minutes in cats, and up to three times that long in people, Laurent says.
The dragons’ eyes flickered, too, during their REM-like sleep. This is a convincing sign that lizards sleep like mammals, Shein-Idelson concludes.
There are doubters
Some scientists disagree, however. They say more evidence is needed to prove that the dragons truly experience two-state sleep.
“It is possible that the dragons were immobile with eyes closed — but awake,” says John Lesku. He’s a comparative-sleep researcher at La Trobe University in Melbourne, Australia. He was not involved in the study. REM brain activity looks similar to that of an awake brain, he notes.
Neuroscientist Niels Rattenborg of the Max Planck Institute for Ornithology in Seewiesen, Germany, also was not part of the new study. Eye movements and limb twitching in REM sleep can look similar to brief awakenings, he points out. More evidence could help resolve whether the dragons are truly snoozing, he says. For example, when a lizard seems to be in REM sleep, waking it should be difficult.
Laurent says that slumbering lizards did show other evidence of sleep. They had less muscle activity and lower heart rates. Researchers could often move the animals without waking them. And the team saw gradual changes in brainwave patterns over the course of a night. For example, sleep cycles got longer. This would be unexpected if the lizards were actually waking up.
If lizards do experience two-stage sleep, it probably comes from a common ancestor of all reptiles, birds and mammals, Laurent says. That small, lizard-like ancestor would have lived some 300 million to 320 million years ago, he says. Back then, all of Earth’s continents were stuck together in one giant landmass called Pangaea
An even older creature could be the true founder of two-state sleep, Laurent says. But to support this idea, researchers would need to find the same sleep patterns in mammals’ more distant relatives, such as fish or amphibians.
These evolutionary clues could be good for neuroscience, Lesku says. “By studying how brain rhythms evolved, we might better understand the function these rhythms play in different types of animals, including ourselves.”
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brainwave An electrical signal produced through the coordinated activity of billions of neurons in the brain of an animal. When charted, the signal typically looks wavy or spiky.
evolution A process by which species undergo changes over time, usually through genetic variation and natural selection. These changes usually result in a new type of organism better suited for its environment than the earlier type. The newer type is not necessarily more “advanced,” just better adapted to the conditions in which it developed.
evolve (adj. evolving) To change gradually over generations, or a long period of time. In living organisms, the evolution usually involves random changes to genes that will then be passed along to an individual’s offspring. These can lead to new traits, such as altered coloration, new susceptibility to disease or protection from it, or different shaped features (such as legs, antennae, toes or internal organs). Nonliving things may also be described as evolving if they change over time. For instance, the miniaturization of computers is sometimes described as these devices evolving to smaller, more complex devices.
landmass A continent, large island or other continuous body of land.
neuroscience Science that deals with the structure or function of the brain and other parts of the nervous system. Researchers in this field are known as neuroscientists.
ornithology The scientific study of birds. Experts who work in this field are known as ornithologists.
Pangaea (or Pangea) The supercontinent that existed from about 300 to 200 million years ago and was composed of all of the major continents seen today, squished together.
REM sleep A period of sleep that takes its name for the rapid eye movement, or REM, that occurs. People dream during REM sleep, but their bodies can’t move. In non-REM sleep, breathing and brain activity slow, but people can still move about.
slow-wave sleep Also known as deep sleep or non-REM sleep. This is the largely dreamless stage of sleep when a person or animal gets the most rest.