Overview
- A 2026 systematic review of EEG research found that caffeine lowers slow-wave activity and shifts brain signals toward a more wakeful pattern even when total sleep time stays the same.
- Slow-wave sleep is the deep phase linked to physical repair and brain waste clearance, so its reduction on EEG indicates less restorative nighttime recovery.
- People vary widely in response to caffeine because genetics, metabolic rate, age, stress, and cumulative daily intake change how long caffeine stays active in the body.
- Timing and dose matter: for some individuals morning coffee can still affect night sleep if the body has not metabolized caffeine, creating a cycle of daytime stimulation and poorer recovery.
- The review signals a shift in sleep science from counting hours to measuring brain function with EEG, which could lead to personalized guidance for workers, students, athletes, and clinicians.