The stimulating properties of caffeine make it an unwise choice for nighttime consumption—unless you’re cramming for an exam or writing an annual report and need to stay awake. A new study from neurobiologists at the University of Montreal (Canada) has revealed how caffeine affects the brain during sleep.
Brain Stimulation Before Sleep – A Barrier to Rest
Caffeine increases the complexity of brain signals and pushes the brain into a state of criticality. That means the brain starts processing information more effectively, especially for learning and decision-making.
But that state interferes with restful sleep. Caffeine doesn’t just keep us alert; it changes how the brain functions. The effect was stronger in people aged 20 to 27.
This impact primarily affects slow oscillations of electrical activity — delta, theta, and alpha waves — which are indicators of deep, restorative sleep. Caffeine weakens these waves, especially during slow-wave sleep (a deep non-REM stage), which the brain uses to consolidate memories and restore cognitive function.
“These changes indicate that even during sleep, the brain remains in a more active but less restorative state under the influence of caffeine. The alteration in rhythmic brain activity helps explain why caffeine affects its recovery efficiency throughout the night,” said Karim Jerbi, the lead author of the study.
How Was the Study Conducted?
The team involved 40 volunteers in their research. Over two nights, they studied the participants’ brain patterns using electroencephalograms (EEG). On the first night, participants received a placebo, while on the second night, they were given a capsule containing caffeine. The capsule contained 200 milligrams of caffeine, equivalent to one or two cups of coffee.
Using statistical analysis, the researchers found a shift toward criticality and increased neuronal excitation.
“While this state is beneficial for concentration during the day, it hinders nighttime rest. The brain cannot relax and recover properly,” said Julie Carrier, a co-author of the study.
Why Does Caffeine’s Effect on the Brain Depend on Age?
The team suggested the effect stems from age-related changes in the brain.
Throughout the day, adenosine molecules gradually accumulate in the brain, producing a growing sense of fatigue as bedtime approaches. Caffeine blocks the receptors that adenosine binds to, providing a temporary energy boost. Younger brains tend to have more adenosine receptors, which makes people in their 20s more sensitive to caffeine’s effects.
The study, published in the journal Communications Biology, points out that caffeine is a psychoactive stimulant consumed daily across age groups in products such as coffee, tea, chocolate, soft drinks, energy drinks, and some pharmaceuticals. Understanding how caffeine affects the sleeping brain—and how those effects change with age—matters for anyone who relies on it to get through the day.
