It’s well known that sleep combats tiredness and is essential for thinking and feeling your best. But why exactly do you need sleep? And how does sleep affect your brain? A recent study sheds light on the role of ‘criticality’ in answering these important questions about sleep.

Criticality is a term used across scientific fields to describe a state in which systems are at their most flexible, and capable of carrying out complex tasks. In physics, criticality is the moment a system is perfectly balanced at the brink of a major change, an instant between order and chaos.

In the brain, scientists propose that networks of brain cells called neurons achieve criticality at the tipping point between bursts of electrical activity. In essence, criticality is a moment when activity in your brain is balanced and unimpeded and your ability to process information is at its peak. 

By monitoring the brain activity of young rats, researchers at Washington University in St. Louis sought to find evidence that restoring your brain to criticality may be one of the main functions of sleep.

Researchers measured brain activity during both wakefulness and sleep, calculating how far the rats’ brains were from a state of criticality at different points over 10 to 14 days. 

The study found several interesting patterns. During waking hours, they found that brain activity moved progressively further from criticality, but only when the rats spent time under lights. 

Sleep, on the other hand, appeared to bring the brains closer to criticality.

Similar to resetting a computer to improve its performance, these findings suggest that sleep may help return the brain to a balanced state for optimal functioning the next day. This reinforces the idea that sleep is an active process that is necessary for your health, not merely a passive state of rest.

While this study opens exciting possibilities about the role of sleep, it’s important to note its limitations. Conducted on young rats, the research leaves questions about how these findings might translate to people. Also, criticality is a difficult theory to prove with certainty, so continued research is needed to deepen our understanding of how and why we sleep.

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4 Sources

  1. National Institute of Neurological Disorders and Stroke. (2023, July 19). Brain basics: Understanding sleep., Retrieved January 28, 2024, from
  2. Scarpetta, S., Morisi, N., Mutti, C., Azzi, N., Trippi, I., Ciliento, R., Apicella, I., Messuti, G., Angiolelli, M., Lombardi, F., Parrino, L., & Vaudano, A. E. (2023). Criticality of neuronal avalanches in human sleep and their relationship with sleep macro- and micro-architecture. iScience, 26(10), 107840.
  3. Xu, Y., Schneider, A., Wessel, R., & Hengen, K. B. (2024). Sleep restores an optimal computational regime in cortical networks. Nature Neuroscience.
  4. Irani, M., & Alderson, T. H. (2023). Tuning criticality through modularity in biological neural networks. The Journal of Neuroscience, 43(33), 5881–5882.

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