Effects of caffeine use and ingestion on a protracted visual vigilance task.

College students (12 female, 12 male) were assigned to either higher caffeine user (HCU) or lower caffeine user (LCU) groups based on a caffeine usage survey. Prior to testing, subjects ingested either placebo or 195 or 325 mg caffeine. They then performed a visual vigilance task measuring response blocks, discrete responses (hits and false alarms), reaction times, and a Mood Check List using a double-blind design. HCU made significantly fewer hits, more false alarms, and also responded faster than LCU. No significant main effects of caffeine administration were found. In the mood analyses, male subjects were more anxious at the end of the experiment. Overall, the user factor (HCU versus LCU) was the most potent experimental variable.