Unraveling the Intricacies of Sleep’s Fundamental Role

Sleep, often regarded as a fundamental physiological necessity akin to food and water, has intrigued researchers for years. Keith Hengen, an assistant professor of biology at Washington University in St. Louis, and a team of researchers from the Arts & Sciences department, have devised a groundbreaking theory that bridges concepts from physics and biology to elucidate the profound purpose of sleep and the intricate workings of the brain.

Their study, published in Nature Neuroscience, posits that sleep serves as a means for the brain to regularly reset its operating system, reaching a state termed “criticality,” which optimizes cognitive functions and processing. 

Hengen likens the brain to a biological computer, emphasizing that memory and experiences during wakefulness gradually alter its code, pulling the system away from an ideal state. The central argument of the study proposes that sleep’s primary function is to restore the brain to an optimal computational state. 

The concept of criticality in physics, existing at the tipping point between order and chaos, provides a framework for understanding the brain’s complex system. Ralf Wessel, a professor of physics, notes that physicists have contemplated criticality for over 30 years.

Criticality describes a system that teeters on the edge between complete regularity and total randomness, maximizing the encoding and processing of information, making it an attractive principle for neurobiology. In a 2019 study, Hengen and Wessel established that the brain actively maintains criticality. 

Challenging the conventional belief that sleep replenishes mysterious and unknown chemicals depleted during wakefulness, the team provides the first direct evidence that sleep restores the brain’s computational power. This is a radical departure from traditional viewpoints, signaling a paradigm shift in understanding the role of sleep. 

To test their theory on the role of criticality in sleep, researchers tracked the spiking of many neurons in the brains of young rats during their normal sleeping and waking routines. Neural avalanches, also known as cascades, were followed through the neural network, representing how information flows through the brain.

At criticality, avalanches of all sizes and durations can occur. As the rats woke up from restorative sleep, avalanches of all sizes were observed. Throughout waking periods, the cascades shifted towards smaller sizes, providing a predictive indicator of the proximity of sleep. The results suggest that every waking moment pushes relevant brain circuits away from criticality, and sleep helps reset the brain. 

The collaboration between physics and biology in this study marks a significant multidisciplinary effort. Initially developed to understand piles of sand on a checkerboard-like grid, the concept of criticality in physics found unexpected application in the study of sleep. In physics, if thousands of grains are dropped on a grid following simple rules, piles quickly reach a critical state, leading to cascades or avalanches. Wessel draws parallels between these neural avalanches and the avalanches of sand on a grid, both serving as hallmarks of systems reaching their most complex state. 

In this analogy, each neuron is likened to an individual grain of sand following basic rules, essentially acting as on/off switches. The collective behavior of billions of neurons reaching criticality creates a state that maximizes complexity, showcasing the brain’s intricate and efficient functioning. Criticality, positioned between excessive order and chaos, optimizes various features crucial for efficient brain function. 

The study represents a significant leap in understanding sleep’s fundamental purpose, integrating principles from physics and biology. By unveiling the brain’s reset mechanism and highlighting criticality as a guiding principle for optimal functioning, the research challenges traditional perspectives on sleep.

This collaboration between physics and biology not only expands our knowledge of the brain’s complexity but also opens new avenues for further investigations into sleep and cognitive functions. The study’s findings may pave the way for future research focusing on the role of sleep in maintaining the delicate balance of the brain’s computational system, pushing the boundaries of scientific inquiry. 

Journal Reference  

Yifan Xu et al, Sleep restores an optimal computational regime in cortical networks, Nature Neuroscience (2024). DOI: 10.1038/s41593-023-01536-9.