That late-night cup of coffee might be doing more harm than you think. A new study from Montreal shows that caffeine doesn’t just make it harder to fall asleep — it actually changes how your brain behaves during sleep, especially in younger people. Researchers discovered that caffeine keeps the brain in a more active, “awake-like” state even when you’re sleeping, preventing it  from fully recharging.

In the study, 40 healthy adults who took either a placebo or the caffeine equivalent to two cups of coffee before bed. Brain scans showed that those  who had caffeine did sleep, but their brains stayed busier than normal. This effect was most noticeable in younger adults, who seem more sensitive to caffeine’s effects.

Experts warn that this brain activity isn’t helpful — it may interfere with memory consolidation and the repair processes that happen during deep sleep. While caffeine might help you stay up late, it could leave your brain feeling wired long after you’ve gone to bed.