Sleep or Die: The Mystery of Why Humans Need Sleep May Have Finally Been Solved

A team of researchers studying the brains of rats says that sleep appears to reset the brain’s operating system to retain a state of optimized performance that computer scientists and physicists call “criticality.”

According to the researchers behind this proposal, this optimized state of consciousness appears so critical that if one goes long enough without sleep, it can lead to serious health consequences, including death.

The Need for Sleep Has Continued to Remain a Medical Mystery

Everyone needs to sleep, but no one is exactly sure why. Of course, sleep is so critical that “you’ll die without it,” said Keith Hengen, an assistant professor of biology at Washington University in St. Louis.

Scientists have proposed many theories, including the relatively common assumption that sleep replenishes certain unknown chemicals needed to keep the brain operating properly. Unfortunately, no one has been able to identify these mystery chemicals, leaving the answer to why humans need to sleep unanswered.

Now, a team of researchers melding the sciences of physics and biology say they may have finally cracked the code. If correct, their research could open up a number of treatments for sleep disorders while simultaneously answering one of the most enduring mysteries in biological science.

Optimizing Brain Performance Via Criticality

In physics, criticality describes a complex system that exists at the tipping point between order and chaos. It’s something computer scientists have been thinking about for decades, but until now, there has been no research connecting it to sleep.

Then, in 2019, Professor Hengen and his colleague, physics professor Ralf Wessel, showed that the human brain actively works to maintain criticality. This result caused the pair to wonder if everyday activities while awake were pushing the brain away from criticality and that the need for sleep was simply the body’s way of restoring this state, much like resetting a computer when it starts to malfunction.

“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.”

To test their idea, the researchers observed the electrical spiking of the brain’s neurons in rats as the animals went about their regular sleeping and waking routines. “You can follow these little cascades of activity through the neural network,” Hengen said.

“At criticality, avalanches of all sizes and durations can occur,” Hengen added. “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.”

Sure enough, the researchers were able to observe criticality as it happened in the rat brains. This included the brain achieving its highest level of criticality right after restorative sleep, as well as its tendency to move away from criticality the longer the lab rats were awake. In fact, the effects of wakefulness on criticality were so significant that the researcher said they were able to predict when the rats would go to sleep based only on their reduced criticality.

“The results suggest that every waking moment pushes relevant brain circuits away from criticality, and sleep helps the brain reset,” Hengen said.

A Beautiful Collaboration Between Physics and Biology

Published in the journal Nature Neuroscience, the team’s research has been confined to rat brains, meaning the results may not be the same for the human brain. Still, the researchers involved sound convinced that they are on the right path.

“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.”

Further testing, including on human subjects, is likely needed to confirm the idea. Either way, along with the study’s tantalizing results, the team’s novel process looks like it may have finally cracked the code on the mystery of sleep once and for all.

“It’s a beautiful collaboration between physics and biology,” Wessel said.

Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at, or email him directly at