Caffeine is everywhere — not just in coffee, but also in tea, chocolate, energy drinks, and many soft drinks. It’s the most widely used psychoactive substance in the world, valued for its ability to boost alertness and energy.
But while caffeine’s daytime benefits are well known, scientists are still learning how it affects the brain during sleep. A new study from the Université de Montréal reveals that caffeine can change brain activity overnight and may interfere with how well the brain recovers.
The research, published in Nature Communications Biology, was led by Philipp Thölke at the Cognitive and Computational Neuroscience Laboratory (CoCo Lab) and co-led by psychology professor Karim Jerbi.
They worked with sleep expert Julie Carrier and her team at the Centre for Advanced Research in Sleep Medicine. Using electroencephalography (EEG) to measure brain activity, along with artificial intelligence to analyze the data, the researchers investigated how caffeine changes the sleeping brain.
For the first time, they showed that caffeine increases the complexity of brain signals and raises the brain’s “criticality” during sleep — and that these changes are more pronounced in younger adults.
Criticality describes a state of balance between order and chaos in brain activity. Jerbi compares it to an orchestra: too quiet, and nothing happens; too chaotic, and the sound turns to noise. At the right balance, the brain is both organized and flexible, able to process information efficiently and adapt.
During the day, caffeine pushes the brain toward this high-performance state, which can help with focus. But Carrier warns that this same effect at night may not be good for rest: “The brain would neither relax nor recover properly.”
The study involved 40 healthy adults who spent two nights in a sleep lab. On one night, they took caffeine capsules three hours and one hour before bedtime; on another night, they took placebo capsules. The researchers recorded their brain activity throughout the night using EEG.
The data showed that caffeine made brain signals more complex and less predictable, especially during non-rapid eye movement (NREM) sleep. This stage of sleep is crucial for restoring the brain, consolidating memories, and supporting learning.
Caffeine also reduced slower brain waves like theta and alpha — which are linked to deep, restorative sleep — and increased beta waves, which are more common when the brain is awake and engaged.
These changes suggest that even during sleep, caffeine keeps the brain in a more active, less restful state. This could interfere with the brain’s ability to recover fully and process memories.
The researchers also found that caffeine’s effects were stronger in younger adults aged 20 to 27 compared to middle-aged adults aged 41 to 58. The difference may come from adenosine receptors in the brain.
Adenosine is a chemical that builds up during the day and makes you feel tired. Caffeine works by blocking these receptors, but as people age, the number of receptors decreases, making caffeine less potent.
This means younger brains may be more sensitive to caffeine’s stimulating effects at night. Given how common caffeine use is — especially among people who drink it to fight fatigue — the researchers say it’s important to understand how it affects brain activity differently across age groups.
They also note that more research is needed to see how these brain changes affect memory, daily functioning, and long-term cognitive health. In the future, findings like these could guide more personalized advice about how much caffeine is safe to consume and when to avoid it.
This study is important because it shows that caffeine’s impact goes beyond just keeping you awake — it can change how the brain functions during sleep. By increasing brain activity complexity and reducing deep-sleep brain waves, caffeine may stop the brain from fully resting and recovering.
The fact that younger adults are more affected suggests that age should be a factor in caffeine guidelines. These results support the idea that timing your caffeine intake — not just limiting how much you drink — may protect your sleep and brain health.
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