How does coffee affect our brain in the evening?

Late caffeine consumption keeps our brain awake even while we sleep

A wake-up call even while we sleep: If we drink coffee late in the evening, it has consequences for our brain. A study shows that caffeine alters our sleep and keeps our brain more alert at night. This hinders the recovery of our brain and could also disrupt nighttime memory formation. Researchers found that this stimulating coffee effect is more pronounced in younger people than in older people.

Caffeine affects nighttime brain activity.

Caffeine is the most commonly consumed psychoactive substance in the world. This natural stimulant is found not only in coffee, but also in tea, chocolate, energy drinks, and many soft drinks. Due to its stimulating and invigorating effects, caffeine consumption can be helpful in many everyday situations because it increases our performance and concentration. Studies also suggest that caffeine is beneficial for memory.

Anyone who has ever drunk coffee late in the day knows that caffeine can make it difficult to fall asleep and keep you awake for an undesirably long time. But what exactly happens in our bodies when we finally do fall asleep?

How does coffee affect sleep?

Researchers led by Philipp Thölke from the University of Montreal have now investigated in more detail how evening caffeine consumption affects sleep. They used an electroencephalogram (EEG) to record the brain activity of 40 sleeping test subjects on two different nights. The first time, the subjects took a capsule containing 100 milligrams of caffeine three hours and one hour before bedtime. On the second night, they received a placebo at the same time.

The researchers then evaluated the EEG data and compared the nighttime brain activity of each participant with and without a caffeine boost in the evening. "We used advanced statistical analyses and artificial intelligence to identify subtle changes in neuronal activity," says Thölke.

Caffeine Makes Nighttime Brain Activity More Complex

The comparison revealed: "Caffeine increased the complexity of brain signals and led to more dynamic and less predictable neuronal activity," reports Thölke. This altered brain activity occurred primarily during the NREM sleep phase – the deep sleep phase crucial for memory formation and mental recovery. It is only during this night's sleep that our brain "sorts" the day's impressions, stores important information in long-term memory, and recalibrates its synapses.

In addition, different rhythms were observed in the electrical brain waves during sleep: On the caffeine-influenced nights, slower oscillations such as theta and alpha waves were weaker, and beta waves were more pronounced. The former are associated with deep, restorative sleep, while the latter occur more frequently during wakefulness and mental activity.

Awakening Effect Even During Sleep

"These changes in brain activity suggest that the brain remains in a more active, less restorative state even during sleep under the influence of caffeine," says senior author Karim Jerbi from the University of Montreal. "This could explain why the brain recovers less well during sleep under the influence of caffeine, with possible consequences for memory processing."

According to the team, this shows that caffeine not only keeps us awake during the day – it exerts its awakening effect even during sleep: "Caffeine stimulates the brain, putting it into a state where it is more alert, vigilant, and reactive. While this is useful for concentration during the day, this state can interfere with rest at night: The brain cannot relax or properly recover," adds co-author Julie Carrier from the University of Montreal.

Young brains respond more strongly to caffeine

The nighttime wake-up effect was more pronounced in younger test subjects between 20 and 27 years of age than in older participants between 41 and 58 years of age. During so-called REM sleep – the sleep phases in which we dream – the brains of the younger participants showed even higher activity. In deep sleep, however, no age differences were found.

Thölke and his colleagues attribute this primarily to the fact that young adults have more receptors for the neurotransmitter adenosine in their brains. This gradually accumulates in the brain over the course of the day and contributes to the feeling of tiredness. Caffeine blocks these receptors and thus keeps us awake. However, the density of adenosine receptors decreases with age. This could explain why the caffeine effect was less pronounced in older people in the study. In addition, the age-related differences could also be due to the fact that older people metabolize caffeine more slowly than younger people, as the team explains.

Individualized recommendations for coffee consumption?

Given these findings and the ubiquity of coffee, Thölke and his colleagues now want to further investigate how and why evening caffeine doses affect the brains of individuals. They hope to be able to provide personalized recommendations for coffee consumption based on age, health status, and time of day. Follow-up studies will also clarify how the observed caffeine-related changes affect brain health in the short and long term. (Communications Biology, 2025; doi: 10.1038/s42003-025-08090-z)

Source: University of Montreal