I drink coffee everyday (somedays four or more espressos). I also stop drinking coffe for a few days at least twice every semester, just in case.
Even at peak coffee, more coffee still stimulates my mind and makes me infinitely more focused. The only times coffee hasn't worked is when I'm genuinely sleep deprived, although I typically get 6-8 hours of sleep.
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.
I hesitate to contribute anecdotal evidence but my experience has been very similar. In college, I routinely took 200 mg caffeine pills and I would always stop cold turkey after exams, when I was usually hitting about 300 mg a day. Definitely felt the alertness almost every time I consumed more caffeine and, surprisingly, never had any withdrawal symptoms outside of drowsiness.
Even at peak coffee, more coffee still stimulates my mind and makes me infinitely more focused. The only times coffee hasn't worked is when I'm genuinely sleep deprived, although I typically get 6-8 hours of sleep.