I would not conclude an effect of caffeine ingestion. You may be getting the alertness boost from placebo effect, conditioned response or even just the fact of

I would not conclude an effect of caffeine ingestion. You may be getting the alertness boost from placebo effect, conditioned response or even just the fact of taking a break. I get an alertness boost from getting up to fill a glass of water, for example. I don't have to actually drink from the glass to get the effect.