Key to the riddle of sleep may be linked to peptidoglycan
The answer to why we sleep might not lie solely in our brains, but in the trillions of microscopic organisms living in our intestines. New research from Washington State University has uncovered compelling evidence that bacterial cell wall fragments naturally present in mouse brains fluctuate in sync with sleep-wake cycles, suggesting our microbial residents play a far more active role in regulating slumber than previously imagined.
The discovery centers on peptidoglycan, a mesh-like component found in bacterial cell walls that has long been known to promote sleep when injected into laboratory animals. Until now, scientists assumed this substance couldn’t naturally migrate to the brain. PhD candidate Erika English and her team proved otherwise, finding peptidoglycan levels rise and fall throughout different brain regions in patterns that mirror natural sleep rhythms.
Bacterial Fragments Follow Sleep Patterns
The researchers measured peptidoglycan concentrations across multiple brain areas at different times of day, discovering the lowest levels occurred at the transition from rest to wake periods. The brainstem showed the highest concentrations, while the olfactory bulb, hypothalamus, and cortex contained lower but still significant amounts.
When mice were deprived of sleep for three to six hours, their brain peptidoglycan levels shifted dramatically. After three hours of sleep disruption, levels increased in the brain’s outer layer but decreased in deeper regions. Extended six-hour deprivation reversed this pattern, causing increases in the brainstem and olfactory bulb.
“This added a new dimension to what we already know. Sleep really is a process. It happens at many different speeds for different levels of cellular and tissue organization and it comes about because of extensive coordination.”
The findings challenge traditional brain-centric models of sleep regulation and support what English and longtime WSU sleep researcher James Krueger call the “holobiont condition” hypothesis. This theory proposes that sleep emerges from constant communication between our neurological systems and the vast community of microorganisms residing within us.
Ancient Origins of Modern Sleep
The implications extend beyond simple sleep mechanics into questions of evolution and consciousness itself. Krueger, recognized as a “Living Legend in Sleep Research” by the Sleep Research Society, suggests our sleep patterns may have originated billions of years ago with basic bacterial activity cycles.
“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.”
This perspective turns conventional neuroscience on its head. Rather than sleep being purely a top-down brain function, the research suggests it may be partially bottom-up, driven by organisms whose evolutionary needs helped shape animal behavior. The bacteria that colonize our guts don’t just passively inhabit us – they may actively influence when and how we sleep.
The research team also examined how sleep deprivation affects gene expression in brain tissue, finding significant changes in genes involved in detecting and responding to bacterial cell wall components. One gene, Pglyrp1, which codes for a peptidoglycan receptor protein, showed particularly strong increases after sleep loss.
These molecular changes align with established knowledge about sleep’s relationship with immune function. Sleep deprivation is known to increase inflammation and susceptibility to infection, while bacterial infections typically cause increased sleepiness. The new findings suggest these connections may be mediated by the constant molecular dialogue between our brains and our bacterial residents.
The discovery adds to mounting evidence that gut microbes influence far more than digestion. Recent studies have linked intestinal bacteria to appetite, mood, cognitive function, and even personality traits. This research suggests we should add sleep regulation to that growing list.
For sleep disorder treatments, the findings hint at entirely new approaches. Rather than focusing solely on brain chemistry or sleep hygiene, future therapies might target the microbial communities themselves. Understanding how bacterial cell wall fragments influence sleep could lead to treatments that work with our body’s natural microbial partners.
The research represents a significant shift in how scientists think about consciousness and control. If our sleep patterns are partially governed by bacterial activity, it raises profound questions about free will and the nature of biological decision-making. Are we truly in control of our rest and wakefulness, or are we responding to biochemical signals from organisms that have been evolving for billions of years longer than mammals?
English acknowledges that much work remains to fully understand these microbe-host interactions. The current findings are primarily observational, establishing correlations rather than proving causation. Future research will need to demonstrate whether manipulating bacterial peptidoglycan levels directly affects sleep patterns.
The study also opens questions about individual differences in sleep patterns and disorders. Variations in gut microbiome composition between people might explain why some individuals are natural early risers while others struggle with morning alertness. Sleep disorders like insomnia could potentially stem from disrupted communication between brain and bacterial systems.
As the scientific community continues to unravel the complex relationships between human physiology and microbial residents, one thing becomes clear: we are not autonomous beings making independent biological decisions. Instead, we exist as composite organisms, our most fundamental behaviors emerging from an ancient partnership between mammalian neurology and bacterial evolution.
The next time you feel drowsy, consider that the signal might be coming not just from your brain, but from the trillions of microscopic partners that call your body home.
Frontiers in Neuroscience: 10.3389/fnins.2025.1608302
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