What is the Mystery Behind Sleep?

In an attempt to unravel the riddle of sleep, scientists monitored the brain activity of sleeping rats and discovered that the brain needs to periodically reset its operating system to achieve “criticality,” a state that optimizes thinking and processing. This was the first concrete proof that sleep restores the computational power of the brain, explaining both the purpose of sleep and the complexity of the brain. Hengen and a team of Arts & Sciences researchers have constructed this theory and published in Nature Neuroscience (1^✔ ✔Trusted Source
Sleep restores an optimal computational regime in cortical networks

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Restorative Power of Brain During Sleep

Sleep is a fundamental need, just like food or water. “You’ll die without it,” said Keith Hengen, an assistant professor of biology at Washington University in St. Louis. But what does sleep actually accomplish? For years, the best researchers could say is that sleep reduces sleepiness — hardly a satisfying explanation for a basic requirement of life.

“The brain is like a biological computer,” Hengen said. “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.”

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,” Wessel said.

Criticality maximizes the encoding and processing of information, making it an attractive candidate for a general principle of neurobiology. In a 2019 study, Hengen and Wessel established that the brain actively works to maintain criticality.

It’s a radical departure from the long-held assumption that sleep must somehow replenish mysterious and unknown chemicals depleted during waking hours.

After their 2019 paper, 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,” Hengen said. “Sleep is a systems-level solution to a systems-level problem.”

Insights on Neural Avalanches that Regulate Sleep

To test their theory on the role of criticality in sleep, the researchers tracked the spiking of many neurons in the brains of young rats as they went about their normal sleeping and waking routines. “You can follow these little cascades of activity through the neural network,” Hengen said.

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These cascades, also called neural avalanches, reflect how information flows through the brain, he said. “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.”

As predicted, avalanches of all sizes occurred in the rats that had just woken up from restorative sleep. Across the course of waking, the cascades started to shift toward smaller and smaller sizes. 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.

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When physicists first developed the concept of criticality in the late 1980s, they were looking at piles of sand on a checkerboard-like grid, a scenario seemingly far removed from brains. But those sand piles provided an important insight, Wessel said.

If thousands of grains are dropped on the grid following simple rules, the piles quickly reach a critical state where interesting things start happening. Avalanches both large and small can start without warning, and piles in one square start spilling into the others. “The whole system organizes itself into something extremely complex,” he said.

The neural avalanches taking place in the brain are much like the avalanches of sand on a grid, Wessel said. In each case, the cascades are the hallmark of a system that has reached its most complex state.

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.

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. “Criticality maximizes a bunch of features that sound very desirable for a brain,” Hengen said.

Reference:

1. Sleep restores an optimal computational regime in cortical networks - (https://www.nature.com/articles/s41593-023-01536-9)

Source-Eurekalert