Caffeine can make sleep lighter even when people fall asleep on schedule and sleep through the night, according to a systematic review of 32 human sleep studies. The review, by researchers at the University of Szczecin and Wroclaw Medical University, found the most consistent effect was a drop in slow-wave activity, the brain rhythm most closely tied to deep, restorative sleep. It was published in Nutrients on April 13 and drew renewed attention after Wroclaw Medical University publicized it on September 22.
The findings matter to anyone who drinks coffee, tea, soda, or energy drinks. Feeling rested may not fully reflect what the brain did overnight. Still, the review measured brain-wave changes, not long-term health outcomes, and it does not change federal caffeine guidance.
