Caffeine is our favourite 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.