Caffeine is one of the most common substances people use to stay awake and alert. Most people know it’s in coffee, but it’s also found in tea, chocolate, energy drinks, and many soft drinks.
Because so many foods and drinks contain caffeine, people all over the world consume it regularly, often without thinking much about how it affects their brain, especially during sleep.
A recent study from researchers at Université de Montréal in Canada offers new insights into what caffeine does to the brain during sleep. The study was published in Communications Biology and led by Philipp Thölke and psychology professor Karim Jerbi, in collaboration with sleep expert Julie Carrier.
Together, their teams used artificial intelligence and brain wave tracking to study how caffeine changes brain activity at night.
The researchers focused on what happens in the brain when people consume caffeine close to bedtime. They found that caffeine makes brain signals more complex during sleep. This means the brain is more active and less calm, which could interrupt the natural recovery process that usually happens during rest. These changes were especially noticeable in young adults.
To understand this better, it helps to know about a concept called “criticality.” One of the researchers, Professor Jerbi, explained that the brain works best when it’s in a balanced state—not too quiet and not too chaotic.
He compared it to an orchestra: if it’s too quiet, there’s no music, but if everyone plays randomly, it becomes noise. A good balance means the brain can learn, make decisions, and adapt quickly. Caffeine seems to push the brain into this active state, even during sleep, when it should be resting.
Julie Carrier added that while this alert state is helpful during the day—like when you’re trying to focus—it can be a problem at night. If the brain is too alert while you’re asleep, it may not rest or recover as it should.
To test how caffeine changes the brain during sleep, the researchers invited 40 healthy adults to participate in a sleep study. Each person took part in two nights of sleep monitoring.
On one night, they took caffeine pills three hours and one hour before bed. On the other night, they took fake pills (placebos). The scientists used a machine called an EEG (electroencephalogram) to record their brain activity.
The results showed that after caffeine, the brain’s activity became more complex and unpredictable, especially during a stage of sleep called non-REM sleep. This is the stage that helps people recover mentally and store memories.
The study also found that caffeine reduced slower brain waves like theta and alpha waves, which are usually linked with deep, restful sleep. At the same time, it increased faster beta waves, which usually show up when people are awake or thinking hard.
This means that even though the participants were asleep, their brains stayed in a more active state after caffeine. According to Professor Jerbi, this could explain why caffeine makes it harder for the brain to fully recover overnight, which might affect memory and learning the next day.
Another important part of the study was how age played a role. The researchers noticed that young adults between 20 and 27 years old were more affected by caffeine than people aged 41 to 58. The younger group had more changes in their brain activity, especially during REM sleep—the part of sleep linked with dreaming.
The scientists believe this difference has to do with a molecule called adenosine. During the day, adenosine builds up in the brain and makes people feel sleepy. Caffeine works by blocking adenosine, helping people feel more awake.
But as people get older, their brains naturally have fewer adenosine receptors. That means caffeine has less of an effect on older adults, which might explain why middle-aged participants didn’t show as many changes.
This study suggests that younger people may need to be more careful about when and how much caffeine they consume, especially before bed. Since caffeine is used by so many people around the world to fight tiredness, it’s important to understand how it affects brain health and sleep in different age groups.
In summary, the study shows that caffeine can make the brain stay active during sleep, reducing the deep rest the brain needs. This is especially true for young adults, whose brains are more sensitive to caffeine.
The researchers suggest that people should think more carefully about their caffeine habits and that more research is needed to understand how these brain changes affect health and daily life.
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The study is published in Communications Biology.
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