Importantly, none of these effects are permanent. Half the caffeine taken into the body is gone, on average, every five hours in a process known as the half-life of elimination. All of it is gone in a day.

Once the body eliminates the caffeine, adenosine once again exerts its physiological effects. As soon as caffeine is gone from the brain, for instance, the blood vessels become dilated by adenosine. This increased blood flow can cause a "caffeine withdrawal" headache -- another thing college students who try to purge themselves of caffeine can tell you all about.