Australian dragons reveal new clues about the mysteries of sleep 

Sleep is one of biology’s enduring mysteries. Present across nearly all animal groups, from jellyfish and insects to birds and mammals, sleep is marked by reduced movement, muscle relaxation, and an increased need to make up for lost sleep. 

But despite its prevalence, scientists still don’t fully understand how sleep rhythms are controlled. 

New research at the Max Planck Institute for Brain Research is shedding light on these mechanisms in an unexpected source: the Australian dragon lizard (Pogona vitticeps). 

The findings, published in the journal Nature, reveal that sleep in reptiles may share ancient roots with mammals and birds, and could help unravel the evolutionary history of sleep.

Sleep states in the Australian dragon 

In humans and many animals, sleep is divided into two primary states: slow-wave sleep (SWS) and rapid eye movement sleep (REMS). 

Slow-wave sleep, characterized by slow brain activity, typically begins sleep cycles, followed by rapid eye movement sleep, which shows brain activity levels similar to wakefulness and includes rapid eye movements and muscle twitches. 

This alternation of sleep states is called the ultradian rhythm, which varies across species. In humans, for instance, a full SWS-REMS cycle lasts about 1 to 1.5 hours, while in the Australian dragon, each cycle lasts just one minute.

Eight years ago, researchers at Max Planck, led by director Gilles Laurent, discovered REM-like sleep in the Australian dragon lizard. 

This discovery suggested that REMS could be an ancient trait shared across amniotes – a group that includes reptiles, birds, and mammals – dating back roughly 320 million years. 

The research team, including postdoctoral scientists Lorenz Fenk and Luis Riquelme, explored why this ultradian rhythm cycles so quickly in reptiles and what drives these alternating states.

Central pattern generator

To investigate further, the team looked for signs of a central pattern generator (CPG), a neural circuit that produces rhythmic motor outputs like walking or breathing. CPGs are well-known in motor control, but their potential role in regulating sleep was novel. 

“This idea of a sleep CPG was completely counterintuitive because CPGs control motor output, whereas sleep is characterized by the near absence of motor activity,” Laurent noted.

To test their hypothesis, the researchers introduced external cues, such as short bursts of light, to see if they could reset the lizards’ sleep cycles – a phenomenon known as phase-dependent reset. 

This is similar to “tripping on a stone while walking,” explained Riquelme, where the rhythm is affected by the timing of the disruption. 

The team found that brief light pulses to the lizards’ closed eyes reset their REM-SWS cycle, pointing to the influence of a CPG-like mechanism in regulating these sleep states.

Partial independence in sleep rhythms

Another discovery revealed that the lizards’ sleep rhythms could be manipulated even while they were awake. This indicates that the circuits driving sleep – and the transition between SWS and REMS – might function independently. 

“This is important because it suggests that sleep and the alternation between SWS and REMS are at least partially independent,” Fenk noted.

Further, the study found that although SWS and REMS alternation occurs on both sides of the brain, only one side can be disrupted, causing it to fall out of sync with the other. 

After a one-sided perturbation, the two hemispheres quickly re-synchronize, suggesting two interconnected CPGs, one for each side of the brain.

Understanding the evolution of sleep

The findings open intriguing questions about the evolution and flexibility of sleep regulation across species. 

According to the scientists, these findings are exciting since they link neural circuits usually associated with motor activity to the regulation of sleep states when the body is at rest. 

Such circuits, likely based in the brainstem, may be adaptable enough to explain the diverse sleep patterns seen in animals, but more research is needed to determine their exact composition and function.

The presence of a CPG-like mechanism in lizard sleep suggests that these circuits may exist across other vertebrates, such as birds and mammals. 

This could mean that sleep has deeper evolutionary roots than previously understood, potentially stretching back to the earliest amniotes. 

The team’s research may even help answer one of the most fundamental questions in biology: why sleep evolved in the first place.

Restorative functions of rest

The study raises significant questions for future research. Could a similar CPG mechanism be found in mammals or birds? If so, how might it account for the different sleep cycle durations seen in these groups? 

And what about animals with vastly different sleep requirements, like dolphins, which can sleep with one hemisphere of the brain at a time?

Understanding the precise role of these circuits in sleep regulation could ultimately offer insights into the purpose of sleep itself. 

While researchers have some understanding of sleep’s restorative functions, the exact benefits remain elusive. Studying how different species control and experience sleep may shed light on why it persists across the animal kingdom, even in vastly different forms and durations.

Studying sleep in the Australian dragon

The study’s results could also change how scientists approach the study of sleep in general. CPGs, which were once thought to be limited to controlling physical actions, may now be central to understanding cognitive and physiological processes that occur when the body is at rest. 

“This discovery suggests that sleep and other complex biological rhythms may be regulated by neural circuits we typically associate with movement,” Fenk said.

These findings lay the groundwork for new research into the role of CPGs in sleep, potentially reshaping our understanding of sleep across diverse animal groups. 

For now, the Max Planck team’s work offers an exciting glimpse into the evolutionary origins of sleep, highlighting that even in rest, the brain is remarkably active, controlled by rhythms that may date back millions of years. 

As researchers continue to uncover the mysteries of sleep, the study of the humble Australian dragon lizard offers a valuable clue about the complexities that lie within the seemingly simple act of sleep.

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