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Late-night scrolling isn’t the only thing sabotaging your slumber. New research suggests that a common daily habit may send your brain into overdrive while you sleep — even if you do it

hours before bedtime. Scientists warn the disruption could interfere with the brain’s overnight recovery processes, potentially taking a toll on your cognitive health. In the study, Canadian

researchers had 40 healthy adults spend two nonconsecutive nights in a sleep lab. On one night, participants consumed 200 milligrams of caffeine — the equivalent of about two cups of coffee

— a few hours before bed. On the other, they were given a placebo. EXPLORE MORE While most people know caffeine can make it harder to fall asleep, researchers used EEG scans to track brain

activity after participants dozed off and found it kept their brains in a heightened state of alertness long after they shut their eyes. They found, for the first time, that the stimulant

pushed the brain into a state of “criticality,” making it more awake, alert and reactive than it should be while catching Zzz’s. “While this is useful during the day for concentration, this

state could interfere with rest at night: the brain would neither relax nor recover properly,” said Dr. Julie Carrier, a psychology professor at the University of Montreal and co-author of

the study. The team used artificial intelligence to detect subtle changes in neuronal activity and found that caffeine increased the complexity of brain signals, preventing the brain from

fully powering down during sleep. The effect was especially strong during non-REM sleep — the deep stage critical for memory and cognitive recovery. Researchers also observed changes in

brain wave patterns. Slow waves linked to deep, restorative rest were reduced, while faster waves associated with wakefulness and mental activity increased. “These changes suggest that even

during sleep, the brain remains in a more activated, less restorative state under the influence of caffeine,” said Dr. Karim Jerbi, a psychology professor at the University of Montreal and

researcher at the Mila Quebec Artificial Intelligence Institute, who co-authored the study. “This change in the brain’s rhythmic activity may help explain why caffeine affects the efficiency

with which the brain recovers during the night, with potential consequences for memory processing,” he added. Notably, the stimulant’s effect’s were far more pronounced in participants aged

20 to 27 compared to those aged 41 to 58 — especially during REM sleep, the dreaming phase tied to emotional and cognitive processing. Researchers believe younger adults responded more

strongly due to having more adenosine receptors. These molecules gradually accumulate in the brain throughout the day, triggering fatigue. “Adenosine receptors naturally decrease with age,

reducing caffeine’s ability to block them and improve brain complexity, which may partly explain the reduced effect of caffeine observed in middle-aged participants,” Carrier said. The

findings suggest younger brains may be especially vulnerable to caffeine’s hidden impact on rest. Caffeine is the most widely consumed psychoactive drug in the world, commonly found in

products such as coffee, tea, chocolate, sodas and energy drinks. A 2023 Sleep Foundation survey found that 94% of US adults consume caffeinated beverages, and 64% drink them daily. Of

those, 40% said they didn’t think it affected their sleep. While the FDA says up to 400 milligrams of caffeine per day is generally safe for healthy adults, the study suggests it could still

be interfering with sleep quality — even hours after your last sip. The scientists are calling for further research exploring how these nighttime brain changes affect day-to-day functioning

and cognitive health. This, they said, could one day shape personalized caffeine recommendations.