For centuries, sleep has been considered a function governed solely by the brain. However, new research from Washington State University (WSU) turns this idea on its head, revealing a surprising communicator in the process: the bacteria living within us.
The study, led by PhD candidate Erika English, found that peptidoglycan (PG)—a fundamental building block of bacterial cell walls—is not only present in the brain but its levels rise and fall in sync with natural sleep patterns. This finding is pivotal because PG was previously known to induce sleep when injected artificially, but its natural presence in the brain was unrecognized.
A New "Holobiont" Theory of Sleep
This discovery lends significant weight to a revolutionary hypothesis developed at WSU, which frames sleep as a "holobiont" condition. A holobiont is a host (like a human) and all of its resident microbes acting as a single, collaborative unit. The theory proposes that sleep emerges from the complex dialogue between the body's neurological systems and its vast microbiome.
"It's not one or the other, it's both. They have to work together," said English. "Sleep really is a process... it comes about because of extensive coordination."
This model bridges two existing theories: the classic brain-centered view and the concept of "local sleep," where individual cells and tissues throughout the body enter sleep-like states. The accumulation of these local signals, influenced by microbial molecules like PG, ultimately tips the entire organism into sleep.
Evolutionary Roots and Future Implications
The implications are profound. It suggests that our sleep-wake cycles may have evolutionary roots stretching back billions of years to the activity cycles of bacteria.
"We think sleep evolution began eons ago with the activity/inactivity cycle of bacteria, and the molecules that were driving that are related to the ones driving cognition today," said WSU Regents Professor James Krueger, a co-author of the study.
This paradigm shift opens exciting new avenues for treating sleep disorders. Instead of targeting only brain chemistry, future therapies might focus on modulating the gut microbiome or its signaling molecules to promote natural, healthy sleep. As research into the gut-brain axis accelerates, our understanding of health, behavior, and even consciousness continues to be transformed.