Caffeine alters brain activity during sleep, making it harder to rest properly, according to a new study by researchers at the University of Montreal. The study found that caffeine increases the complexity of brain signals and pushes the brain toward a state called “criticality,” where it balances between order and chaos. This state is ideal for learning and decision-making when awake but may disrupt restful sleep.
The researchers monitored 40 healthy adults over two nights using electroencephalograms (EEGs). On one night, participants took a placebo; on the other, they consumed 200 milligrams of caffeine—about the amount in one or two cups of coffee—before bedtime. The caffeine caused the brain to remain more active during sleep, especially in the non-rapid eye movement (NREM) phase, which is crucial for memory consolidation and cognitive recovery.
Caffeine weakened slow brain waves—delta, theta, and alpha—that are linked to deep, restorative sleep. Instead, it increased beta wave activity, which is typical of wakefulness. This shift suggests that caffeine keeps the brain in a less restorative and more alert state overnight, reducing the brain’s ability to recover and process memories effectively.
The effect was strongest in younger adults aged 20 to 27. Scientists believe this is because younger brains have more adenosine receptors, which caffeine blocks to produce its stimulating effects. As people age, the number of these receptors declines, making older adults less sensitive to caffeine’s impact on sleep.
Neuroscientist Karim Jerbi, co-author of the study, explained, “Even during sleep, caffeine keeps the brain more activated and less restorative.” Julie Carrier, a sleep specialist involved in the research, added, “While caffeine helps concentration during the day, it prevents the brain from relaxing and recovering properly at night”.
Given caffeine’s widespread use in coffee, tea, energy drinks, chocolate, and medications, the researchers emphasize the importance of understanding its effects on sleep and brain function across different ages.
The study was published in Nature Communications Biology in April 2025.
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