• Posted June 1, 2025

How Caffeine Affects Your Brain While You Sleep

SUNDAY, June 1, 2025 (HealthDay News) — Your morning cup of coffee might help you power through the day — but could it be keeping your brain too alert at night, even while you sleep?

A new study suggests that caffeine doesn’t just affect your energy levels. It may also change how your brain functions during sleep, especially in 20-somethings.

Researchers in Canada found that drinking caffeine before bed may keep the brain in a more active state overnight, a news release shows. 

Published in the journal Nature Communications Biology, the study used electroencephalography (EEG) and artificial intelligence to examine how caffeine changes the brain’s behavior during rest.

"We used advanced statistical analysis and artificial intelligence to identify subtle changes in neuronal activity," said lead author Philipp Thölke, a research trainee at the University of Montreal.

"The results showed that caffeine increased the complexity of brain signals, reflecting more dynamic and less predictable neuronal activity, especially during the non-rapid eye movement (NREM) phase of sleep that's crucial for memory consolidation and cognitive recovery,” he said in a news release.

Researchers found that caffeine increased the complexity of brain signals. It also boosted something known as "criticality."

“Criticality describes a state of the brain that is balanced between order and chaos,” said study co-author Karim Jerbi, a University of Montreal psychology professor and director of its Cognitive and Computational Neuroscience Lab. 

“In this state, the brain functions optimally," he said in a news release. "It can process information efficiently, adapt quickly, learn and make decisions with agility.”

But during sleep, that same alert state isn't exactly ideal.

“Caffeine stimulates the brain and pushes it into a state of criticality, where it is more awake, alert and reactive,” study co-leader Julie Carrier, director of the Canadian Sleep and Circadian Network, explained in a news release. “While this is useful during the day for concentration, this state could interfere with rest at night: the brain would neither relax nor recover properly."

To study these effects, researchers observed 40 healthy adults as they slept. Participants took either caffeine capsules or placebos on different nights, three hours before bedtime, and then one hour before bedtime.

The researchers found that caffeine increased beta brain waves — usually associated with mental engagement and wakefulness — and weakened slower brain waves like theta and alpha, which are tied to deep and restorative sleep.

"These changes suggest that even during sleep, the brain remains in a more activated, less restorative state under the influence of caffeine," Jerbi said.

"This change in the brain's rhythmic activity may help explain why caffeine affects the efficiency with which the brain recovers during the night, with potential consequences for memory processing,” he addd.

The impact of caffeine was especially strong in 20- to 27-year-olds, compared to middle-aged adults between 41 and 58.

This difference may be due to changes in adenosine receptors in the brain. Adenosine is a chemical that builds up throughout the day, making you feel sleepy. 

Caffeine blocks adenosine receptors to keep you awake — but younger adults have more of these receptors, so the stimulant has a stronger effect.

"Adenosine receptors naturally decrease with age, reducing caffeine's ability to block them and improve brain complexity, which may partly explain the reduced effect of caffeine observed in middle-aged participants," Carrier said.

Since caffeine is so widely used — especially as a daily solution for fatigue — researchers said further study is needed to understand its long-term effects on brain health and to guide personalized recommendations for different age groups.

More information

The Sleep Foundation has more on caffeine and sleep problems.

SOURCE: University of Montreal, news release, May 29, 2025