EEG studies suggest caffeine can reduce deep, restorative sleep even when sleep duration appears normal. Its effects vary by individual, and relying on caffeine for energy may come at the cost of nighttime recovery.
Evening coffee has long been debated. Some people can drink it and fall asleep with no trouble, while others struggle to sleep afterward. But a growing body of research suggests that focusing only on whether coffee delays sleep may miss a more important issue: what caffeine does to the brain during sleep.
Researchers are increasingly using EEG, or electroencephalography, to study caffeine’s effects on sleep. This technique records the brain’s electrical activity, allowing scientists to examine not only how long people sleep or how often they wake up, but also the underlying quality of their sleep.
“EEG allows us to see not only whether a person is sleeping but also how the brain is sleeping. 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,” explains Prof. Donata Kurpas from the Department of Nursing, Wroclaw Medical University.
Slow waves are a defining feature of deep sleep, the stage that supports physical recovery, replenishes energy stores, and helps maintain healthy brain function.
Deep Sleep Quality May Suffer Despite Normal Sleep Time
Studies show that caffeine’s effects do not always appear as shorter sleep or difficulty falling asleep. In many cases, the bigger impact is on the quality of sleep.
“Caffeine may shorten sleep or make it more difficult to fall asleep; however, even when sleep duration appears normal, it may reduce slow-wave activity and shift the EEG pattern toward a more ‘wakeful’ brain,” says Prof. Kurpas.
As a result, someone may spend eight hours in bed while their brain receives less of the restorative benefits of deep sleep. Many people may not realize this is happening.
“The subjective feeling of having slept well does not always correspond to what we observe in neurophysiological recordings. A person may fall asleep without major difficulty and not remember awakenings, while the brain may display fewer features of deep sleep,” the expert adds.
Why Caffeine Affects People Differently
One of the clearest findings from sleep research is that people respond to caffeine very differently. Genetics, metabolism, age, stress, and chronic fatigue can all influence its effects.
For some people, even a morning cup of coffee may affect sleep later that night.
“It is not only about coffee consumed just before bedtime. For some people, the total amount of caffeine consumed during the day and whether the body has enough time to metabolize it before nightfall may also be important,” Prof. Kurpas emphasizes.
This information is especially relevant for people who perform demanding mental work, athletes, and anyone who regularly uses caffeine to boost focus and performance.
The Hidden Cost of Borrowing Energy
Caffeine increases alertness and helps reduce feelings of fatigue. However, experts note that this boost may sometimes come at the expense of overnight recovery.
“If caffeine helps a person function during the day while simultaneously worsening the quality of nighttime recovery, a vicious circle may develop: greater fatigue, greater need for stimulation, and poorer sleep,” says Prof. Kurpas.
Because of findings like these, sleep researchers are paying less attention to sleep duration alone and focusing more on how the brain functions during sleep.
“Caffeine is neither ‘good’ nor ‘bad’. It is a biologically active substance whose effects depend on dose, time of day, age, lifestyle, sleep quality, stress burden, and individual sensitivity,” the expert concludes.
Reference: “The Caffeinated Brain Part 2: The Effect of Caffeine on Sleep-Related Electroencephalography (EEG)—A Systematic and Mechanistic Review” by James Chmiel and Donata Kurpas, 12 April 2026, Nutrients.
DOI: 10.3390/nu18081220
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