New research reveals that molecules from gut bacteria are found in the brain, fluctuating with our sleep-wake cycle and challenging everything we thought we knew about slumber.
For centuries, the question of what causes us to sleep has been directed at one primary suspect: the brain. We’ve viewed sleep as a top-down command, a neurological master switch flicked by complex processes within our own heads. But what if the command isn’t coming from inside the control tower? What if it’s being broadcast from the trillions of microscopic tenants living in our gut?
New, paradigm-shifting research from Washington State University (WSU) suggests that our understanding of sleep is about to get a lot more crowded. Scientists have discovered that molecules from the cell walls of common bacteria are not only present in the brain but their levels rise and fall in direct correlation with our sleep-wake cycle. This finding builds a powerful bridge between the brain-centered model of sleep and the emerging science of the microbiome, suggesting our microbes could be as fundamental to slumber as our neurons.
A Bacterial Molecule in the Brain
The key to this discovery is a substance called peptidoglycan (PG). If you imagine a bacterium, peptidoglycan is a crucial component of the mesh-like, structural wall that gives the cell its shape and integrity. For a long time, scientists knew that injecting PG into animals could induce sleep, but the prevailing wisdom was that these bacterial molecules couldn’t naturally cross the highly selective blood-brain barrier to reach the brain on their own.
The WSU research team, led by PhD candidate Erika English, decided to challenge that assumption. In a study recently published in Frontiers in Neuroscience, they demonstrated that not only is peptidoglycan naturally present in the brains of mice, but so are the specific receptor molecules that allow the brain to detect and communicate with it. Even more compelling, the concentration of these bacterial molecules changed with the time of day and was significantly affected by sleep deprivation. It appears our brains are not only aware of these bacterial components but are actively listening to them.
"This added a new dimension to what we already know," explained English, highlighting the significance of finding a direct molecular link between the microbial world and the central nervous system’s sleep regulation.
The "Holobiont Condition": A New Theory of Sleep
This groundbreaking finding is a cornerstone of a broader hypothesis proposed by English and longtime WSU sleep researcher Professor James Krueger in the journal Sleep Medicine Reviews. They call it the "holobiont condition" of sleep. A holobiont is an entity made up of a host (like a human) and its many resident species of microbes. This new hypothesis posits that sleep isn’t governed by one system, but arises from the constant, complex communication between the host’s body and its vast microbial community.
This idea elegantly merges two previously competing theories of sleep. The first is the traditional, brain-centric model where neurological systems dictate when we sleep and wake. The second is the concept of "local sleep," which suggests that sleep isn’t an all-or-nothing event for the whole brain. Instead, it’s the accumulation of tiny sleep-like states in small networks of cells throughout the body. As more of these cellular networks go offline—like individual lights winking out in a house at night—the entire organism tips from wakefulness into sleep.
The holobiont hypothesis suggests it’s not one or the other. "It’s not one or the other, it’s both. They have to work together," English stated. Sleep is a coordinated process, a symphony played by both our own cells and the bacteria we host, from the smallest cellular level to the whole organism.
Are We in Charge? Microbes and the Bottom-Up Brain
The implications of this research extend far beyond the bedroom. It contributes to a growing body of evidence that is turning our traditional view of cognition on its head. For decades, we’ve assumed a top-down model of control: the brain makes a decision, and the body follows. But the link between the microbiome and behavior suggests a powerful bottom-up influence.
Our gut microbes, it seems, may have a say in our cognition, appetite, sex drive, and other fundamental behaviors. This doesn’t just challenge our understanding of neurology; it touches on philosophical questions about free will. If the needs of the tiny organisms living inside us can influence our actions, who is truly the host and who is the guest?
Professor Krueger, who was named a "Living Legend in Sleep Research" in 2023, places this relationship in an evolutionary context. "We have a whole community of microbes living within us. Those microbes have a much longer evolutionary history than any mammal, bird or insect – much longer, billions of years longer," he noted. He theorizes that the very origins of sleep may lie in the ancient activity-inactivity cycles of bacteria. The molecules that drove those primitive cycles, he suggests, are related to the ones that help drive our own cognition and sleep today. In a sense, animals may have evolved as sophisticated hosts, shaped by the evolutionary pressures of their resident microbes.
The Future of Sleep
This work begins to provide a concrete mechanism for previously observed, but poorly understood, connections. We already knew that our sleep patterns can alter the health of our gut microbiome and, conversely, that bacterial infections often make us sleep more. The discovery of peptidoglycan in the brain offers a direct pathway for this communication.
As we continue to unravel this intricate dialogue between our bodies and our bacteria, the possibilities are immense. This research could pave the way for entirely new classes of treatments for sleep disorders, potentially targeting the microbiome to help regulate the brain. It’s a thrilling frontier in science, reminding us that we are not solitary beings but complex, walking ecosystems.
As English concluded, "Now that the world has come to appreciate how important microbes are, not just for disease but also for health, it’s a very exciting time to start to expand on our understanding of how we are communicating with our microbes and how our microbes are communicating with us."
References
English, E. A., & Krueger, J. M. (2025). Peptidoglycan and its receptors are present in the murine brain and vary with sleep-wake state. Frontiers in Neuroscience.
English, E. A., & Krueger, J. M. (2025). The holobiont condition: A new paradigm for sleep regulation. Sleep Medicine Reviews.
