Unbeknownst to most, there are significant differences between light- and dark-roasted coffee including the difference in the caffeine levels, the flavor characteristics and the quality of the coffee.
Light-roasted coffee exhibits more of the inherent flavor characteristics of the beans, or the origin where the beans were grown, as well as more acidity or brightness. Dark-roasted coffee tends to take on the full-bodied flavor of the roasting process and is less acidic. Sometimes lower quality beans are roasted very dark to mask defects or impurities in the beans. In extremely dark roasts, it is tough to distinguish between high and low quality beans.
As coffee is roasted darker, the beans lose water weight but grow larger in size. During this expansion, oils trapped inside the beans migrate toward the surface. If the beans are roasted dark enough, the oils will reach the outer surface of the bean and give off that oily sheen that we all recognize as dark roasted coffee.
Caffeine content in dark-roasted coffee compared to light-roasted coffee is almost the same. Any difference is minuscule. Here's where it gets interesting, because dark-roasted coffee has less mass than light-roasted coffee, when measuring by weight (which is how you should ALWAYS measure coffee for brewing), more dark-roasted coffee is needed to equal the same weight of light-roasted coffee. Since more coffee is being used to brew, a darker roast will result in a cup with a higher caffeine content.