Drinking coffee daily may lead to shallow sleeping, according to a new study which warns that caffeine can weaken brain activity linked to recovery and rejuvenation. Even if you think you have slept for eight hours, your brain may not have fully regenerated.
Quality Over Quantity
A growing body of research shows that the quality of sleep matters more than the duration, especially for the brain. Studies reveal that slow brain waves are a key component of “deep sleep”, the phase responsible for physical rejuvenation and proper cognitive function. Caffeine, the new study suggests, may negatively affect deep sleep.
“Caffeine may shorten sleep or make it more difficult to fall asleep,” said Donata Kurpas, an author of the study published in the journal Nutrients. “However, even when sleep duration appears normal, it may reduce slow-wave activity and shift the EEG pattern towards a more wakeful brain.”
This means that even when regular coffee-drinkers spend eight hours in bed, their brain may not fully regenerate. “A person may fall asleep without major difficulty and not remember awakenings while the brain may display fewer features of deep sleep,” Dr Kurpas, from Wroclaw Medical University, added.
Individual Variability
The effects of coffee can vary widely among people, depending on their genetics, metabolic rate, age, stress, and chronic fatigue levels. While it can increase alertness and reduce the sensation of fatigue, its effects may sometimes resemble borrowing energy at the expense of night-time regeneration.
“For some people, the total amount of caffeine consumed during the day and whether the body has enough time to metabolise it before nightfall may be important,” Dr Kurpas said. “If caffeine helps a person function during the day while simultaneously worsening the quality of night-time recovery, a vicious circle may develop: greater fatigue, greater need for stimulation and poorer sleep.”
EEG Insights
The researchers used EEG to get insights into the brain's electrical activity. “EEG allows scientists to see not only whether a person is sleeping, but also how the brain is sleeping,” Dr Kurpas explained. “Classical sleep assessment measures sleep duration and its stages, whereas quantitative EEG analysis reveals more subtle changes, such as reduced slow-wave activity.”
The team collected and assessed data from 32 studies conducted between 1980 and 2026 to explore the connection between caffeine exposure or administration and sleep-related outcomes. They consistently found that caffeine was linked to a suppression of low-frequency deep sleep activity, particularly slow-wave brain activity. It produced a “lighter, more aroused, and more wake-like sleep EEG profile,” the study said. “These effects were especially prominent during early-night NREM sleep and in recovery sleep after sleep deprivation.”
The study’s findings suggest that regular coffee-drinking may shift sleep dynamics “towards a more excitation-dominant state”. The researchers hope further studies are conducted, prioritising larger and more diverse samples of participants to clarify the real-world, functional consequences of caffeine-induced EEG changes.
