Coffee pushes sleeping brain to critical state

A new study shows that caffeine not only keeps you awake—basically, the way the brain works during sleep changes, pushing neural activity toward a hyperactive “critical state” that may interfere with the recovery process essential for memory and cognitive recovery.

Using artificial intelligence and brain monitoring techniques, researchers found that caffeine increases brain complexity and transfers neurodynamics to an enhanced state of excitation, especially among young people, who respond the most strongly to the world’s most popular psychoactive drug.

This study, published in Communication Biology, challenges our understanding of how caffeine affects the sleeping brain, rather than just making it harder to fall asleep. Scientists at the University of Montreal used EEG to monitor 40 healthy adults over two nights — one taken caffeine capsules before bed, and the other with a placebo — revealing a huge change in brain wave patterns and nerve complexity.

Key points discovery

“Critical description of the state of the brain that balances order and chaos,” explains Karim Jerbi, professor of psychology and director of the UDEM lab for Cognitive and Computational Neuroscience. “It’s like an orchestra: too quiet, nothing happens, too messy, and harsh. Criticality is a medium of joy, and brain activity is both organized and flexible.”

Although this critical state can achieve optimal brain function within the time of waking (allowing effective information processing and rapid adaptation), the presence during sleep may prove problematic. Julie Carrier, professor of sleep psychology at UDEM Advanced Research Center for Sleep Medicine, notes that caffeine “stimulates the brain and pushes it to a critical state where it is more awake, alert and responsive. While this is useful during the day, this state can interfere at night: the brain neither relaxes nor recovers.”

The meaning is more than just a simple sleep destruction. During sleep, especially during the non-specific eye movement (NREM) phase, the brain usually runs in a more ordered, predictable state that promotes memory consolidation and cellular recovery. Caffeine seems to prevent this natural downshift, thus keeping nerve activity stronger when the brain is still.

Machine Learning Reveals Hidden Patterns

Lead author Philipp Thölke is a research trainee at Cocoa Laboratory, who uses advanced artificial intelligence algorithms to detect subtle changes that are invisible to traditional analytical methods. “We used advanced statistical analysis and artificial intelligence to determine subtle changes in neuronal activity,” Thölke explained. “The results show that caffeine increases the complexity of brain signals, reflecting more dynamic and more predictable neuronal activity, especially during the non-specific eye movement (NREM) phase of sleep, which is crucial for memory consolidation and cognitive recovery.”

AI analysis shows that the effects of caffeine go far beyond the well-known reduction in deep sleep. The researchers found that complexity measures, including entropy calculations and LEMPEL-ZIV complexity, are more effective than traditional brain wave analysis to distinguish between caffeine and placebo sleep states. This suggests that the greatest impact of caffeine may be on the fundamental computational properties of neural networks, not just surface-level brain rhythms.

Especially surprisingly, caffeine flattens the brain’s power spectrum, which changes with changes in excitation inhibition balance. During sleep, this transition to increasing nerve excitation represents a fundamental deviation from the brain’s natural nocturnal state, where inhibition processes often dominate to promote rest and recovery.

Age-dependent vulnerability

The study found significant age differences in caffeine sensitivity. Young people aged 20-27 responded more to caffeine than middle-aged participants aged 41-58, especially during REM sleep, which is a phase associated with dreams and emotional processing.

This difference stems from age-related changes in adenosine receptor density. Adenosine is a molecule that accumulates gradually throughout the day to promote drowsiness, binding to specific brain receptors of caffeine disorders. “Adenosine receptors naturally decrease naturally with age, thus reducing the ability of caffeine to block them and improve brain complexity, which may partly explain the effects of caffeine observed in middle-aged participants,” the vector observed.

This finding is of great significance for caffeine consumption recommendations across age groups. While older people may suffer severe sleep damage due to nightly caffeine consumption, young people seem particularly susceptible to caffeine-induced changes in sleep brain dynamics.

Technical breakthroughs in sleep research

The study uses complex signal processing techniques to separate brain activity into periodic (oscillation) and multiple (background) components. This separation suggests that the effect of caffeine is more pronounced when the researchers explain the changes in the brain’s 1/f slope, a measure of the relationship between nerve excitation and inhibition.

Without this correction, the effect of many caffeine on specific brain wave frequencies will be undetectable. This approach represents a significant advance in sleep studies, suggesting that previous studies may have underestimated the effects of caffeine on nerve oscillations without taking these background changes into account.

The researchers also found that caffeine reduced the remote temporal correlation of brain activity—a pattern that often suggests healthy neuroharmony. This reduction suggests that caffeine may disrupt the brain to remain coherent, and synchronous active patterns are necessary for optimal sleep-dependent processes.

Impact on daily life

These findings raise important questions about the timing and dosage of caffeine, especially for young people. Study participants received 200 mg of caffeine—equivalent to 1-2 cups of coffee—three hours before bedtime. Even this modest dose and timing can cause significant changes in brain complexity throughout the night.

How will these findings affect our understanding of sleep quality and cognitive recovery? Studies show that the effects of caffeine go far more than making it more difficult to fall asleep. By keeping the brain more active and less restorative throughout the night, caffeine may interfere with critical processes including memory consolidation, cell repair, and metabolic recovery.

Jerbi highlights the broader significance: “This change in the brain’s rhythmic activity may help explain why caffeine affects the brain’s efficiency of recovery at night, which has potential consequences for memory processing.”

The researchers stressed that further research is needed to understand how these neural changes affect cognitive health and daily functioning. Given the ubiquity of caffeine – found in billions of dollars consumed daily in coffee, tea, chocolate, energy drinks and soft drinks – understanding its complex effects at different ages and health conditions is increasingly important for public health advice.