Caffeine & Your Brain
Why coffee helps, and when it doesn't
Caffeine works by blocking adenosine, a molecule that builds up in your brain throughout the day and makes you feel sleepy. By occupying adenosine receptors without activating them, caffeine stops the sleepy signal from getting through, leaving you feeling more alert. It doesn't add energy. It removes the brake.
Two important catches. First, your brain adapts: regular caffeine drinkers grow more adenosine receptors, which is why your morning coffee eventually feels less like a boost and more like baseline. Second, timing matters. Caffeine has a half-life of around 5 to 6 hours, so a 3 PM coffee is still circulating at bedtime, often disrupting sleep in ways you don't notice but that show up in next-day cognition. The sweet spot for most people is one or two cups in the morning, finishing well before mid-afternoon.
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