[peoplerock]

Former biofeedback therapist here. The effect of caffeine on muscle tension is measurable: it is impossible for nearly everyone to achieve as deep a muscular relaxation (measured by EMG) after just 100mg caffeine.

I haven't looked at evidence in years, but the gastrointestinal and heart rate effects of even higher levels can be remarkable. I remember medical journal articles (Am J Psychiatry, i believe) referring to it as "caffeinism" - since the mental-alertness benefits habituate, but not the autonomic stimulation.

Like with any mind-alteration: exercise caution. Listen to what your own body tells you.