I have been out of work for all but 2 of the last 16 months.  This have given me time to do some experimenting (among other things).  One thing I did was to try to find the best dose of caffeine for me.  I tried regular coffee, decaff, and coffee and caffeine pills; several times and in different orders.  With as close to controlled conditions as one person alone could arrange, I discovered that caffeine has <i>absolutely no effect</i> on me.  I could discern absolutely no difference in alertness, learning (read a textbook chapter and did test at end), or reaction time (simple video game).  I already knew I didn't react strongly to caffeine, hence the experimenting, but the result was a surprise.  Apparently all of the effects I had previously attributed to caffeine had been placebo.