The sleep it is a basic need, just like food or water. “Without it you would die,” said Keith Hengen, assistant professor of biology at Washington University in St. Louis. But what does sleep actually accomplish? Why do we sleep? For years, the best researchers have been able to say is that sleep reduces drowsiness, hardly a satisfactory explanation for a basic requirement of life.
Fusing concepts from the fields of physics and biology, Hengen and a team of Arts & Sciences researchers have built a theory that could explain both why we sleep and the complexity of the brain. As reported in a new study, researchers monitored the brain activity of sleeping rats to demonstrate that the brain needs to regularly reset its operating system to reach “critility,” a state that optimizes thinking and processing.
The results of research were published in Nature Neuroscience.
Why do we sleep? Here is science's answer
“The brain is like a biological computer,” Hengen said. “Memory and waking experience change the code little by little, slowly moving the larger system away from an ideal state. The whole point of why we sleep is to restore an optimal computational state.”
Co-authors of the paper include Ralf Wessel, professor of physics; Yifan Xu, a biology graduate student studying neuroscience; and Aidan Schneider, a graduate student in the Computational and Systems Biology program, all in Arts and Sciences.
Wessel said physicists have been thinking about criticality for more than 30 years, but they never imagined that the work could affect sleep. In the world of physics, criticality describes a complex system that exists at the critical point between order and chaos. “At one extreme, everything is completely smooth. 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 determined that the brain actively works to maintain criticality.
In the new study into why we sleep, the team provides the first direct evidence that sleep restores the brain's computational power. This is a radical shift from the long-held assumption that sleep must somehow replenish the mysterious, unknown chemicals depleted during waking hours.
Following their 2019 paper, Hengen and Wessel theorized that learning, thinking, and being awake must distance the brain from critical issues, and that sleep is perfectly positioned to reset the system. “We realized that this would be a really interesting and intuitive explanation for why we sleep and the main purpose of sleep,” Hengen said. “Sleep is a system-level solution to a system-level problem.”
To test their theory about the role of criticality in sleep and why we sleep, the researchers monitored the spiking of many neurons in the brains of young rats as they went about their normal sleep and wake routines. “You can follow these little cascades of activity through the neural network,” Hengen said.
He said these cascades, also called neural avalanches, reflect how information flows through the brain. “Under critical conditions, avalanches of all sizes and durations can occur. Far from the criticality, the system becomes biased towards 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 expected, avalanches of all sizes occurred in rats newly awakened from restful sleep. Over the course of the awakening, the falls began to move toward smaller and smaller sizes. The researchers found that they could predict when the rats were about to fall asleep or wake up by tracking the distribution of avalanches. When the size of the waterfall was reduced to a certain point, sleep was not far away.
“The findings suggest that each waking moment pushes relevant brain circuits away from criticality, and sleep helps the brain reset,” Hengen said.
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 scenario seemingly far removed from the brain.
But those piles of sand provided important insight into why we sleep, Wessel said. If thousands of beans are thrown onto the grill following simple rules, the piles quickly reach a critical state where interesting things start to happen. Large and small avalanches can begin without warning, and piles in one square begin to spill into others. “The whole system organizes itself into something extremely complex,” she said.
Wessel said neural avalanches that occur in the brain are very similar to sand avalanches on a grid. In any case, waterfalls 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 or not to fire based on direct inputs. If billions of neurons can reach the tipping point – the sweet spot between too much order and too much chaos – they can work together to form something complex and beautiful and make sense of why we sleep.
“Criticality maximizes a set of characteristics that seem very desirable for a brain,” Hengen said.
The new study on why we sleep was a multidisciplinary effort. Hengen, Xu, and Schneider designed the experiments and provided the data, while Wessel joined the team to implement the mathematical equations needed to understand sleep in the criticality framework. “It's a beautiful collaboration between physics and biology,” Wessel said.
Research in rats shows that cortical arousals and brief arousals during sleep exhibit non-equilibrium dynamics and complex organization on time scales necessary for spontaneous transitions between sleep stages and maintenance of healthy sleep. Prof. Plamen chap. Ivanov of Boston University and colleagues present these findings in PLOS Computational Biology.
Why we sleep and the function of sleep are traditionally considered a homeostatic process that resists deviation from equilibrium. In this regard, brief episodes of wakefulness are viewed as disturbances leading to sleep fragmentation and related sleep disorders.
While addressing aspects of why we sleep and sleep regulation related to consolidated sleep and wakefulness and the sleep-wake cycle, the homeostatic paradigm does not take into account the dozens of abrupt transitions between sleep phases and the microstates within the phases. of sleep during the night.
Ivanov and colleagues hypothesized that, while sleep is indeed homeostatic on time scales of hours and days, non-equilibrium dynamics and criticality underlie the microarchitecture of sl
eep on shorter time scales.
To test this hypothesis, the researchers collected electroencephalographic (EEG) recordings of brain activity over multiple days in normal rats and in rats with lesions to the parafacial area, a region of the brain that helps regulate sleep.
The researchers analyzed the explosive dynamics of brain activity patterns known as theta waves and delta waves, which are observed in both sleeping rats and humans and indicate why we sleep.
Their empirical results and models indicate that awakenings from sleep are a manifestation of an intrinsic unbalanced sleep regulation mechanism related to the self-organization of neuronal ensembles. This mechanism acts on time scales of seconds and minutes and remains online through continuous bursts in brain wave rhythms.
The study of why we sleep also suggests that maintaining a critical non-equilibrium state is essential for the flexibility of the sleep regulation system to spontaneously trigger multiple transitions between different sleep phases and between sleep and brief wakefulness throughout the lifespan. sleep period.
This critical state is also necessary for the complex microarchitecture of sleep that is increasingly recognized as a characteristic of healthy sleep. The critical behavior observed in sleep draws parallels with other non-equilibrium systems in critical conditions, such as earthquakes.
“Paradoxically, we find that the 'resting' state of healthy sleep is maintained through bursts in cortical rhythm activity that obey temporal organization, statistical, and mathematical laws similar to those of earthquakes,” Ivanov says. “Our findings serve as building blocks for better understanding sleep and could help improve the detection and treatment of sleep disorders.”
#sleep #sleep
