Here is a fun citation with a brief summary. They suggest regular caffine use lowers your baseline and it just returns you to where you'd be if you weren't dependent.

University of Bristol. "Coffee consumption unrelated to alertness: Stimulating effects may be illusion, study finds." ScienceDaily. ScienceDaily, 3 June 2010. <www.sciencedaily.com/releases/2010/06/100602211940.htm>.

It allows you to rearrange your alertness budget in time if not in total.