In the version of this article initially published, there were errors in the reporting of statistics. In the Figure 1b legend, the asterisked P value was given in the HTML version as *P = 0.05 and in the PDF version as *P < 0.05. It should read *P < 0.05, one-tailed. In the Figure 2a legend, the degrees of freedom for the immediate caffeine group were given as 42 and the P value as 0.05; the correct values are 71 and 0.049, respectively. In the Figure 2b legend and the fifth paragraph of the main text, the P value for the main effect of caffeine was given as 0.001; the correct value is 0.05. The errors have been corrected in the HTML and PDF versions of the article.