Why do we need to sleep?
Keith Hengen, assistant professor in the department of biology at Washington University in St. Louis, delves into our brain to find the answer.
Keith Hengen was born and raised in Concord, NH, and was first introduced to neuroscience at Bates College. From there, Keith went directly to grad school at the University of Wisconsin-Madison where he studied the brains of hibernating ground squirrels. Keith then did his postdoctoral work under the mentorship of Gina Turrigiano at Brandeis University. It was there that he developed a deep interest in the mechanisms by which the brain maintains stable function/computation. Application of these theories to complex networks and behavior inevitably led to criticality and sleep. It’s the intersection of these two ideas that serves as the core of the Hengen Laboratory at Washington University in Saint Louis. The Hengen Lab, which started in 2017, is home to theoretical physicists, chemists, physiologists, computer scientists, a practicing neurologist, and a fleet of young students who are drawn to neuroscience. In addition to sleep, the lab is making great progress in applying ideas like criticality to the study of neurodegenerative diseases, learning, and complex behavior.
Sleep Resets the Brain’s Operating System
“Why do animals need sleep?” is one of the great standing questions in biology. It’s intuitively obvious that we need sleep; almost all cognitive functions degrade when we’re exhausted, and severe sleep deprivation is ultimately fatal. However, despite a long list of theories over the last century, nothing has meaningfully explained the restoration of brain function that is central to a good night’s sleep.
Implied by the restoration of function is the concept of a homeostatic set-point, much like the temperature setting on a thermostat. Recent work has revealed that brains operate around a computational set-point called criticality: imagine adjusting a network of neurons to the most excitable point possible before the network is consumed by seizure-like activity. It’s precisely at this point, just on the edge of chaos, that information processing and problem solving are maximized.
However, this set point is fragile – in models, criticality is quickly undermined by learning and memory, which involve changing the strength of connections between neurons. In other words, normal experience may come at a steep cost to the basic function of the brain. This sparked our realization that the restoration of criticality might be the central function of sleep.
To investigate this, we recorded neurons in the visual cortex of laboratory rats for two weeks. Waking experience pushed the cortex away from criticality, and sleep robustly restored it. Crucially, future sleep and wake were predicted by the how close the brain was to criticality. No other measures of brain activity had this predictive power. Together, this suggests that criticality is consistent with the central set-point around which sleep is operating. These findings have the potential to directly explain sleep’s ability to make us think clearly, operate effectively, and feel like a million dollars.
