For coffee drinkers who insist they can fall asleep after their post-dinner espresso, a new review suggests the more important issue may be the quality of that sleep.
The systematic review from researchers in Poland, published April 13 in the journal Nutrients, found that caffeine can change the brain’s electrical activity during sleep, even if the duration of sleep is largely the same.
The review, authored by James Chmiel of the University of Szczecin and Donata Kurpas of Wroclaw Medical University, included 32 human studies involving caffeine exposure and sleep-related EEG, a method for recording electrical activity in the brain.
Measuring Sleep Depth
The broader finding that caffeine can interfere with sleep was not surprising. What stood out more was the conclusion that caffeine may make sleep less restorative, even when people do not perceive major sleep disruption.
“EEG allows us to see not only whether a person is sleeping, but also how the brain is sleeping,” Kurpas said in an announcement of the publication. “Classical sleep assessment assesses sleep duration and its stages, whereas quantitative EEG analysis reveals more subtle changes, such as reduced slow-wave activity, which is an important marker of sleep depth and its restorative character.”
The review found that in some studies, even relatively low bedtime caffeine doses suppressed slow-wave activity during the first part of the night, when sleep pressure is normally high and deep sleep is most abundant.
Beyond slow-wave suppression, caffeine also increased faster brain activity in the spindle and beta ranges, producing what the authors described as a “lighter, more aroused and more wake-like sleep EEG profile.”
Feelings About Sleep
One of the more counterintuitive findings involved subjective experience. Across multiple studies, participants’ self-reports of sleep quality did not track with EEG evidence of disruption.
“The subjective feeling of having slept well does not always correspond to what we observe in neurophysiological recordings,” Kurpas said. “A person may fall asleep without major difficulty and not remember awakenings, while the brain may display fewer features of deep sleep.”
The effects were especially pronounced in recovery sleep after sleep deprivation — a finding with direct implications for the practice of using caffeine to push through fatigue. In multiple studies, when sleep-deprived participants took caffeine before later sleep, the brain showed a weaker-than-expected rebound in slow-wave activity. In other words, caffeine not only masked tiredness during wakefulness; it also appeared to blunt the brain’s attempt to recover.
The studies reviewed did not all test caffeine at the same time of day. Some examined caffeine taken near bedtime or in the evening, while others looked at daytime use, repeated intake, withdrawal or caffeine taken after long periods awake.
A Research Awakening
Caffeine and sleep — two daily realities for billions of people — continue to form a popular intersection among academic researchers.
One 2025 study from a team at the University of Montreal found that caffeine kept sleeping brains in a more active and less restful state. A 2024 study from a team in Europe suggested repeated caffeine intake may worsen some brain changes tied to sleep deprivation. A 2021 study from authors at Michigan State University found that caffeine may help with attention after sleep loss while failing to restore performance on more complex tasks.
The review was funded by a Wroclaw Medical University grant. The authors declared no conflicts of interest.
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