Our sensitivity to caffeine declines as we age, resulting from a decrease in the number of adenosine receptors in our brains.  Adenosine is a chemical that builds up in our brains throughout the day and makes us feel sleepy.  Taking caffeine blocks these receptors, which makes us feel more alert and awake. However, as we age, the number of adenosine receptors in our brains naturally decreases.  This means that caffeine has less effect on us, and we may need to consume more of it to feel the same level of alertness.