Daniel watches Mom drink her morning coffee and assumes it gives her energy.

It doesn't. Not exactly.

Coffee doesn't add energy to your system. What it does is block the signal that tells your brain it's tired. Those are two very different things -- and understanding the difference changes how you think about coffee, tiredness, and sleep.

The chemical at the center of the story is adenosine -- your brain's tiredness signal. It builds up throughout the day, attaches to receptors in your brain, and tells you it's time to rest. Caffeine works because its molecular shape is similar enough to adenosine that it can slip into the same receptors and take up the space. But it doesn't activate them. It just blocks adenosine from getting in.

So the tiredness is still there. Still building. Your brain just can't feel it.

When th ...