Caffeine can lower deep‑sleep brain waves, making sleep less restorative even when total sleep time appears normal, with effects varying widely among individuals.
Researchers at Wroclaw Medical University used electroencephalography (EEG) to examine caffeine’s impact on sleep. Their measurements showed that caffeine reduces slow‑wave activity, a key indicator of deep, restorative sleep, even when participants fall asleep at their usual time and remain in bed for a full eight‑hour period.
Prof. Donata Kurpas explained that subjective feelings of having slept well may not align with neurophysiological recordings; fewer slow waves suggest shallower sleep. The study highlighted marked individual differences in caffeine sensitivity, influenced by genetics, metabolism, age, stress and chronic fatigue, meaning coffee consumed earlier in the day can still affect nighttime recovery for some people.
The researchers warned that reduced deep‑sleep quality could create a feedback loop: caffeine boosts daytime alertness but may impair nighttime recovery, leading to greater fatigue and potentially increased caffeine use. Kurpas emphasized that caffeine’s effects depend on dose, timing, lifestyle and personal sensitivity rather than being uniformly good or bad.
The reporting is limited to the systematic review and statements from the university team; broader population impacts and long‑term health consequences remain uncertain.