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Sleep restores the balance between order and chaos in our brains

Researchers at Washington University in St. Louis have proposed a new theory explaining the fundamental purpose of sleep

Led by Professor Keith Hengen and supported by a team of experts from various disciplines, the study delves into the complex relationship between sleep and the brain’s operational efficiency.

Optimizing the brain

The research builds upon the concept of “criticality,” a principle commonly discussed in physics but now applied to neurobiology. 

Hengen, along with physicists like Ralf Wessel and biology graduate students Yifan Xu and Aidan Schneider, explored how sleep acts as a reset mechanism for the brain, maintaining its optimum computational state.

Biological computer 

“The brain is like a biological computer,” said Hengen. “Memory and experience during waking change the code bit by bit, slowly pulling the larger system away from an ideal state. The central purpose of sleep is to restore an optimal computational state.”

Hengen highlighted the importance of sleep, comparing it to biological necessities like food and water. “You’ll die without it,” he said, emphasizing its critical role in sustaining life. 

Restoring the balance

The study challenges the long-standing view that sleep merely reduces tiredness, proposing instead that it restores the brain to a state of criticality. 

In this context, criticality refers to a system teetering between complete regularity and randomness, a condition that maximizes the brain’s ability to encode and process information efficiently.

Order and chaos 

Wessel said physicists have been thinking about criticality for more than 30 years, but they never dreamed the work would have implications for sleep. 

In the world of physics, criticality describes a complex system that exists at the tipping point between order and chaos. “At one extreme, everything is completely regular. At the other extreme, everything is random,” said Wessel.

Departure from previous theories 

A 2019 paper by Hengen and Wessel established that the brain actively maintains this state of criticality. The new study takes this idea further by providing direct evidence that sleep is essential in restoring the brain’s computational power. 

This marks a significant departure from previous theories that sleep replenishes certain unknown chemicals depleted during waking hours.

Reset mechanism

Hengen and Wessel theorized that learning, thinking, and being awake must push the brain away from criticality and that sleep is perfectly positioned to reset the system. 

“We realized this would be a really cool and intuitive explanation for the core purpose of sleep,” said Hengen. “Sleep is a systems-level solution to a systems-level problem.”

Neural avalanches 

To validate their theory, the team observed the brain activity of young rats, focusing on neural cascades or avalanches, which are indicative of how information flows through the brain. These cascades vary in size, and their distribution changes with the state of wakefulness or sleep. 

The researchers discovered that a diverse range of cascades occurred right after the rats woke up from restorative sleep, but as wakefulness persisted, these cascades began to skew towards smaller sizes. 

This shift served as a predictor for the onset of sleep, suggesting a direct link between the distribution of neural avalanches and the need for sleep.

“You can follow these little cascades of activity through the neural network,” said Hengen. “At criticality, avalanches of all sizes and durations can occur. Away from criticality, the system becomes biased toward only small avalanches or only large avalanches. This is analogous to writing a book and only being able to use short or long words.”

Intriguing results

The researchers found they could predict when rats were about to go to sleep or wake up by tracking the distribution of avalanches. When cascade sizes were reduced to a certain point, sleep wasn’t far away.

“The results suggest that every waking moment pushes relevant brain circuits away from criticality, and sleep helps the brain reset,” Hengen said.

Criticality in physics

In an interesting parallel, Wessel refers to the concept of criticality in physics, initially observed in piles of sand on a grid. The dynamics of these sand piles, which self-organize into a complex system characterized by spontaneous avalanches of varying sizes, mirror the neural cascades in the brain. 

This similarity underscores the interdisciplinary nature of the study, blending principles of physics with biological phenomena.

Something complex and wondrous

According to Hengen, every neuron is like an individual grain of sand following very basic rules. Neurons are essentially on/off switches that decide whether or not to fire based on straightforward inputs. 

Hengen noted that criticality maximizes a bunch of features that sound very desirable for a brain. If billions of neurons can reach criticality – the sweet spot between too much order and too much chaos – they can work together to form something complex and wondrous, said the researchers.

The study is published in the journal Nature Neuroscience.


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