Researchers are increasingly using EEG (electroencephalography) to investigate how caffeine affects the brain during sleep — and findings suggest the issue goes beyond simply “falling asleep.” According to Prof. Donata Kurpas of Wroclaw Medical University, caffeine may reduce slow-wave activity, a key marker of deep, restorative sleep, even when sleep duration appears normal. EEG analyses reveal that the brain can remain in a more “wakeful” state overnight, potentially impairing recovery despite the subjective feeling of good sleep. Scientists also emphasize strong individual differences in caffeine sensitivity linked to genetics, metabolism, age, stress, and fatigue. The findings suggest that caffeine’s stimulating effects may contribute to a cycle of daytime fatigue and poorer nighttime recovery, highlighting the importance of sleep quality — not only sleep length — in understanding caffeine’s impact on health.