Sleep is, of course, essential to survival. For such a ubiquitous aspect of human existence, however, the “why” has always been considered a grand question in biology, according to Keith Hengen, assistant professor of biology at Washington University. That is, until now.
“There was this guy, Bill Dement, who is considered to be the father of American sleep medicine,” Hengen says. “He was asked, ‘Why do we sleep?’ shortly before he died, and his answer was snarky. He just said, ‘I don’t know—to reduce sleepiness.’”
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It’s a moment in scientific history that stuck with Hengen: “While we know that sleep does lots of things that are good for health and cognition in general, the central feature of why we sleep—and why we feel like a million bucks after 10 hours of uninterrupted sleep—that’s remained unknown. I have always been drawn to that question. It just seems really fundamental and foundational to how brains work, how animals work, how behavior works, how cognition works. So, we took a crack at it.”
After what he calls a back-of-the-napkin lunch chat that pooled insights across departments, Hengen assembled a team of physics and biology researchers to construct a theory explaining the meaning of sleep and its impact on the brain. Co-authors of the paper would eventually include Ralf Wessel, a professor of physics; Yifan Xu, a graduate student in biology studying neuroscience; and Aidan Schneider, a graduate student in the Computational & Systems Biology program, all in Arts & Sciences.
The resulting study, recently published in Nature Neuroscience, involved tracking the brain activity of sleeping rats. The study made the case that sleep functions as a way to regularly “reset” the brain—as if it were an operating system or a computer—to reach “criticality,” or the state that optimizes thinking and processing.
Hengen explains that the brain is more or less a biological computer and that memory and experience during waking hours pull that system away from an ideal state. So, why do we sleep? To restore the brain to that optimal state, Hengen says. “Simply using your brain comes at a cost to its function,” Hengen explains. “In other words, as you move through the world and you experience things—you learn things, you see things, you remember what you ate for breakfast, where you parked your car, a conversation you just have with a co-worker—all of those experiences actually influence the way that the brain can do computation. And so as you accumulate those experiences over the course of the day, you end up pushing yourself further and further away from this optimal set point. So you’re kind of degrading the foundation of the brain by virtue of simply using it. And powerfully, sleep seems to restore that.”
The tools used in this study and the idea of criticality are concepts that physicists and biologists have toyed with for decades—but this study approaches those ideations in a decidedly fresh way. “We are certainly not the first people to suggest that this idea of criticality might be relevant to biological computers, the brain,” he says. “But really beginning to sort of test the application of those ideas in the context of sleep—that’s the big breakthrough here.”
Hengen and his team hope that this study adds context and gravitas to people’s understanding of the important of sleep. “There’s value in simply understanding why we sleep or why some feature of biology is the way it is because that gives you a little context for the risks of say, chronic sleep deprivation,” Hengen says. “I think we all know your risk for cancers goes up and your risk for obesity and diabetes goes up, but that’s on really long timescales, and it’s not a very immediate risk, right? But it’s a very acute and very real thing to suggest that you’re basically running your computer in a faulty state. And to be able to quantify that, hopefully, has value for the average person. [I hope] they come away with an acute understanding of the contribution of sleep to their daily experience. And given that it has so many other health implications, it’s just kind of putting a little more weight on the scale.”
What really excites Hengen and his colleagues about their findings, however, is their potential applications for long-term neurological study.
“Oftentimes, people will look at these really complex dynamics and features of the brain in terms of really acute behaviors—like a lever-pressing task or a maze or an animal eating—on these short timescales,” Hengen says. “I guess I’m drawn to this question about reliability over a lifetime and diseases that unfold over long periods of time.”
That long-term approach might just help in diagnoses and treatment of chronic disease, such as Alzheimer’s and neurodevelopmental disorders, Hengen says. “For instance, what are implications for diseases where it seems that the brain might either not be able to reach the right set point or maintain it? I think this is much bigger than just sleep. For instance, one of the strongest and most universal symptoms of Alzheimer’s disease or neurodevelopmental disorders is broken sleep. I think we’re going to find that these things all come full circle and that they’re all pointing in the same direction.”
