EEG reveals how coffee may secretly lower sleep quality

Evening coffee has sparked controversy for years. Some people fall asleep without difficulty, while others toss and turn for half the night. However, a growing body of research suggests that the question of whether "coffee makes it harder to fall asleep" may be too simplistic. What appears to matter far more is what happens in the brain during sleep. 

Scientists studying the effects of caffeine on sleep are increasingly turning to EEG, or electroencephalography - a method used to record the brain's electrical activity. Thanks to EEG, it is possible to observe not only sleep duration or moments of awakening, but also the biological quality of sleep itself. 

– 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 one of the key components of deep sleep - the phase responsible for bodily regeneration, restoration of energy resources, and proper brain function. 

Caffeine may cause "shallow" sleep 

Research shows that the effects of caffeine do not always manifest as shorter sleep or difficulty falling asleep. Much more often, the changes concern the quality of nighttime rest. 

– 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. 

This means the body may spend eight hours in bed, but the brain may fail to fully regenerate. People are often unaware of this. 

– 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 does coffee affect everyone differently? 

One of the most interesting conclusions emerging from research is the enormous individual variability in response to caffeine. Genetics, metabolic rate, age, stress levels, and chronic fatigue all play a role. 

For some individuals, even coffee consumed in the morning may be problematic. 

– 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 is particularly important information for people engaged in intellectual work, athletes, and anyone who regularly uses caffeine to improve performance and concentration. 

Energy is borrowed from the body 

Caffeine improves alertness and reduces the sensation of fatigue, but experts point out that its effects may sometimes resemble "borrowing energy" at the expense of nighttime regeneration. 

– 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. 

For this reason, modern sleep research is increasingly moving away from simple questions about sleep duration and focusing instead on how the brain functions during nighttime rest. 

– 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.