Someone asked me the other day, "is it true that dark roast coffees have less caffeine than light roast coffees? Or is it the other way around?"
The answer isn't as straightforward as you might like. Either way, the answer can be yes.
When raw coffee is introduced into the roaster, you've got about ten minutes of chemical and physical changes that turn a rock-hard, tasteless seed into the soft, aromatic, and delicious beans we know and love. One of the physical changes is simply evaporation. Not just water though. Caffeine evaporates too.
During the roasting process, coffee is going to lose 10-25% of its weight. Most of that is water because it evaporates at a low temperature compared to the other natural chemicals in the coffee, and that weight directly corresponds to the roast level. A light roast will lose relatively little weight. The darker the roasting goes, the more weight it will lose in the process.
That means dark roast coffees have less caffeine by bean, but light roast coffees have less caffeine by weight. When we make a cup of coffee in our shop, we weigh out 23g of coffee, rather than counting out 57 beans, so chances are, a darker roasted coffee will use more beans to fill 23g, and that's where you end up with more caffeine.
How much of a difference does this make? A negligible amount. Really, variety will play a much bigger role than roast level, but does it matter? We always say the coffee with the most caffeine is the one you want to have a second cup of.
Reasons to Drink Coffee
TL;DR Sort of, yes, but there's not a big enough difference for you to notice. Just drink what you enjoy drinking.