Was in Whole Foods yesterday, not a regular patron as the company is the Nieman Marcus (aka Needless Markup) of grocery stores, but had a gift card so I was there to buy another pound of so of a regional blend (called Chesapeake Bay Blend here) which is really very good.
As I was measuring out the beans for grinding, the section clerk came by and I remarked that of all the beans they have out, this particular blend does seem to move quickly. I was told in return that that is probably due to particular the aroma and full-bodied flavor this bend has which appeals to so many. I also said that despite the very dark beans, the coffee does not 'wire' me very much either.
And what I was told, and the factoid, was that the darker the roasted bean, the less caffeine there should be thus less of a jolt. Seems counterintuitive to me, especially as espresso blends are very, very dark, but she's selling/roasting the stuff so I guess she should know what she's speaks of.