Caffeine is found not only in coffee but also in tea, chocolate, energy drinks, and many soft drinks, making it one of the most widely consumed psychoactive substances in the world.
In a study published in late April in the journal
Nature Communications Biology, a research team from the University of Montreal sheds new light on how caffeine can alter sleep architecture and influence the brain's recovery—both physical and cognitive—during the night.
More specifically, the research team demonstrated for the first time, using artificial intelligence and electroencephalography, that caffeine increases the complexity of brain signals and enhances the "criticality" of neuronal activity during sleep. Interestingly, this phenomenon was more pronounced in young adults.
"Criticality describes a state of the brain that exists in a balance between order and chaos," explains Professor Jerbi. "It can be compared to an orchestra: too calm, and nothing happens; too chaotic, and it's cacophony. Criticality is that sweet spot where brain activity is both organized and flexible. In this state, the brain functions optimally: it can process information efficiently, adapt quickly, learn, and make decisions with agility."
"Caffeine stimulates the brain and pushes it toward a state of criticality, where it is more awake, alert, and reactive," says Professor Carrier. "While useful during the day for focus, this state could harm nighttime rest: the brain may not relax or recover properly."
Disrupted electrical rhythms in the brain
To study how caffeine affects the sleeping brain, Julie Carrier's team recorded the nighttime brain activity of 40 healthy adults using electroencephalography, comparing nights when participants consumed caffeine capsules (three hours and one hour before bedtime) with nights when they took a placebo.
"We used advanced statistical analysis methods and artificial intelligence to detect subtle changes in neuronal activity," mentions Philipp Thölke, the study's lead author. "The results showed that caffeine increased the complexity of brain signals—reflecting more dynamic and less predictable neuronal activity—particularly during non-rapid eye movement (NREM) sleep, a crucial phase for memory consolidation and cognitive recovery."
The team also discovered striking changes in the brain's electrical rhythms during sleep: caffeine dampened slower oscillations like theta and alpha waves—typically associated with deep, restorative sleep—and boosted beta wave activity, which is more common during wakefulness and mental engagement.
"These changes suggest that, even during sleep, the brain remains in a more activated, less restful state under the influence of caffeine," explains
Karim Jerbi, who also holds the Canada Research Chair in Computational Neuroscience and Cognitive Neuroimaging. "This disruption in the brain's rhythmic activity could help explain why caffeine limits how effectively the brain restores itself overnight, with potential consequences for memory processing."
Age matters: caffeine effects more pronounced in young adults
The study also revealed that caffeine's effects on brain dynamics were significantly stronger in young adults—aged 20 to 27—compared to middle-aged participants—41 to 58—especially during REM sleep, the phase associated with dreaming.
Young adults had a stronger response to caffeine, likely due to higher densities of adenosine receptors in their brains. Adenosine is a molecule that gradually accumulates in the brain throughout the day, creating a sense of fatigue.
"Adenosine receptors naturally decrease with age," notes Julie Carrier, "thereby reducing caffeine's ability to block them and enhance brain complexity, which may partly explain the weaker caffeine effects observed in middle-aged participants."
These age-related differences suggest that younger brains may be more sensitive to caffeine's stimulating effects. Given caffeine's global popularity, understanding its complex neural effects across age groups is essential, especially due to its daily use in combating daytime fatigue.
Further research will be needed to clarify how these neural changes influence cognitive health and daily brain function, potentially leading to personalized caffeine consumption recommendations, the researchers conclude.