Not sleeping is incompatible with life, but the reason for its necessity for humans constitutes a mystery that science has been trying to unravel for decades. A team of researchers from Washington University in St. Louis (WashU), in the United States, has developed a theory that could help explain both the purpose of sleep and the complexity of the brain.
Sleep is a basic need, just like food or water. “You’ll die without it,” says Keith Hengen, assistant professor of biology at WashU. But what is the true use of sleep? For years, the best researchers have found is that sleep decreases drowsiness, but this is not a satisfactory explanation for a key requirement for life.
The authors of the new study tracked the brain activity of sleeping rats to show that the brain needs to periodically reset its operating system to reach “criticality,” a state that optimizes thinking and processing. Hengen has compared the brain to a biological computer in which “waking memory and experience modify the code little by little, slowly distancing the larger system from an ideal state. “The central purpose of sleep is to restore an optimal computational state.”
Predict sleep and wake time
Ralf Wessel, professor of physics and co-author of the work, said that physicists have been thinking about criticality for more than 30 years, but never imagined that 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 normal. At the other end, everything is random,” he says.
The study results have been published in Nature Neuroscience and provide the first direct evidence that sleep restores the brain’s computational power. It’s a radical departure from the long-held assumption that sleep must somehow replenish mysterious, unknown chemicals that are depleted during the waking phase.
From a 2019 study by Hengel and Wessel, these scientists theorized that learning, thinking, and being awake should distance the brain from criticality and that sleep is perfectly positioned to reset the system. “We realized that this would be a really interesting and intuitive explanation for the central purpose of sleep,” Hengen said. “Dream is a systems-level solution to a systems-level problem.”
“The results suggest that each moment of wakefulness moves relevant brain circuits away from criticality, and sleep helps the brain reset”
The researchers decided to test their theory about the role of criticality in sleep by tracking the spikes of many neurons in the brains of young rats during their normal sleep-wake patterns. “You can follow these small cascades of activity through the neural network,” explains Hengen. These neural cascades or avalanches reflect how information flows through the brain, she said. “At critical times, avalanches of all sizes and durations can occur. Away from criticality, the system leans towards small or large avalanches. This is similar to writing a book and being able to use only short or long words.”
Just as they had predicted, avalanches of all sizes occurred in the rats that had just woken up from a restful sleep. Throughout the vigil, the waterfalls began to change, adopting smaller and smaller sizes. The researchers found that they could predict when the rats were about to go to sleep or wake up by following the distribution of the avalanches. When the size of the waterfalls was reduced to a certain extent, the dream was not far away. “The results suggest that each moment of wakefulness moves relevant brain circuits away from criticality, and sleep helps the brain reset,” Hengen said.
When physics meets biology
When physicists first developed the concept of criticality in the late 1980s, they were looking at piles of sand on a chessboard-like grid, a very different scenario from the brain, but one that provided important information, according to Wessel. If thousands of grains are dropped onto the grid following simple rules, the piles quickly reach a critical state where avalanches, both large and small, can begin without warning, and piles from one square begin to spread to others. “The whole system is organized into something extremely complex,” he says.
The neural avalanches that occur in the brain are a lot like sand avalanches on a grid, Wessel says. In each case, cascades are the hallmark of a system that has reached its most complex state.
According to Hengen, each neuron is like an individual grain of sand that follows very basic rules. Neurons are essentially on/off switches that decide whether to activate or not based on direct inputs. If billions of neurons can reach the critical point (the sweet spot between too much order and too much chaos), they can collaborate to form something complex and wonderful. “Criticality maximizes a series of characteristics that seem very desirable for a brain,” Hengen concludes.
