Effects of caffeine on vigilance and other performance tests in normal subjects.

In two randomized double-blind crossover studies, 8 and 10 healthy volunteers took either 200 mg caffeine or placebo. Objective and subjective measures of caffeine effects were carried out over the following 1-3.5 h. Auditory vigilance, a test lasting 1 h, showed significantly better performance on caffeine than on placebo. In the second study, this effect was only apparent in the second half of the test. Of the shorter objective tests used, only finger tapping showed a significant effect of caffeine, the rate of tapping over 1 min being increased. The subjective assessments showed increased interest and alertness in the caffeine session. Reliable detection of the effects of mild stimulant drugs using objective measures may require the use of tests of long duration.