Subjective effects of caffeine among introverts and extraverts in the morning and evening.

In previous studies of psychomotor performance, the stimulant effects of caffeine differed by personality characteristics. For example, caffeine improved the task performance of extraverts but overaroused introverts and thus impaired their performance. The present study compared the effects of caffeine on subjective arousal among introverts and extraverts. Seventeen introverts and 19 extraverts drank coffee that contained doses of 0, 2, and 4 mg/kg caffeine during morning and evening sessions in a within-subjects, randomized, double-blind, crossover design. At 30-min intervals for 180 min after drinking, participants completed the Profile of Mood States, a battery of self-report visual analog scales, and the Digit Symbol Substitution Test (DSST). Caffeine effects on mood and task performance did not significantly interact with extraversion, except for nonsignificant trends for caffeine to increase happiness and vigor more among extraverts than introverts. No 3-way interactions of group, time, and dose were found on any scales or on the DSST. Results do not support the hypothesis that caffeine differentially affects extraverts and introverts, particularly at different times of the day.