Caffeine is our favorite drug. But if we miss out on our fix, it can be a real headache, in more ways than one. Caffeine is a stimulant. It quickly enters our brain and blocks the (adenosine) receptors that are responsible for dulling brain activity. By blocking the dulling of our brain, we feel a sense of invigoration, focus and subtle euphoria. These feelings can also enhance our performance of certain focused tasks, like driving or staying awake through the whole lecture. This is the upside of caffeine. The downside is how we feel when we are not getting our usual dose. Because of the anticipated highs of brain activity after our cup, the lows without it seem longer and deeper.