Caffeine is one of the most widely used substances in the world.
It’s in coffee, tea, chocolate, energy drinks, and many soft drinks, making it a daily habit for millions.
Most people know it as the ingredient that helps them feel awake and alert. But what happens when caffeine is still in your system at night?
A new study from the Université de Montréal shows that caffeine doesn’t just keep you up — it changes the way your brain works while you sleep, and it could affect how well your brain recovers overnight.
The study, published in Nature Communications Biology, was led by research trainee Philipp Thölke and psychology professor Karim Jerbi, in collaboration with sleep researcher Julie Carrier and her team.
Using advanced brain monitoring and artificial intelligence, the researchers found that caffeine increases the complexity of brain signals and raises the brain’s “criticality” during sleep — especially in younger adults.
Criticality is a term scientists use to describe the brain’s balance between order and chaos. Imagine an orchestra: if it’s too quiet, nothing happens; if it’s too chaotic, the music turns into noise. The “happy medium” is when all parts work together in a flexible, organized way.
In this state, the brain can process information, adapt, and make decisions effectively. During the day, caffeine can help push the brain toward this state, which is why it can improve focus and alertness. But at night, this alert-like state could stop the brain from fully relaxing and recovering.
To study the effects, the researchers monitored the brain activity of 40 healthy adults while they slept. On one night, participants took caffeine capsules three hours and one hour before bedtime. On another night, they took placebo capsules instead. Brain activity was recorded using an electroencephalogram (EEG), which measures electrical patterns in the brain.
The results were striking. Caffeine made brain signals more complex and less predictable, particularly during non-rapid eye movement (NREM) sleep. This stage is crucial for restoring the brain, consolidating memories, and preparing for the next day.
Caffeine also changed the brain’s rhythms: it reduced slower waves like theta and alpha, which are linked to deep, restful sleep, and boosted beta waves, which are more common when the brain is awake and actively thinking.
These changes mean that even when someone is asleep, caffeine may keep the brain in a more “on” state, reducing the quality of rest and potentially interfering with memory processing and recovery.
The study also revealed a big difference between younger and older adults. People aged 20 to 27 were far more affected by caffeine, especially during rapid eye movement (REM) sleep — the stage when most dreaming happens. This may be because younger people have more adenosine receptors in their brains.
Adenosine is a chemical that builds up during the day and makes you feel tired. Caffeine works by blocking these receptors. As people age, the number of receptors decreases, which could explain why caffeine had a weaker effect in participants aged 41 to 58.
These findings suggest that younger brains may be more sensitive to the stimulating effects of caffeine at night. Since caffeine is so widely consumed, especially as a way to fight fatigue, the researchers say it’s important to understand how it affects the brain across different ages and health conditions.
They also point out that more research is needed to see how these changes in brain activity affect long-term cognitive health, memory, and daily functioning. In the future, the results could help guide personalized advice on how much caffeine is safe to drink — and when to stop drinking it during the day — depending on age and lifestyle.
This study is important because it connects caffeine’s well-known effects on alertness to its less obvious effects on sleep quality and brain recovery. By showing that caffeine keeps the brain in a more active state at night, it helps explain why some people feel less refreshed after sleeping, even if they get enough hours of rest.
The age-related differences also highlight the need for tailored caffeine guidelines, especially for young adults who may be more vulnerable to its sleep-disrupting effects.
If you care about sleep, please read studies about Sleeping pill lowers key Alzheimer’s disease markers and findings of Scientists discover the link between sleep apnea and dementia risk.
If you care about sleep, please read studies about Common sleep supplement could improve memory and findings of Scientists find link between short sleep and higher risk of long COVID.
The study is published in Communications Biology.
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