The effects of caffeine, impulsivity and time of day on performance, mood and cardiovascular function.

Two experiments were carried out to examine the effects of caffeine on performance, mood and cardiovascular function. The results showed that the effects of caffeine depended on the dose, time of administration, the function being examined and the impulsivity of the subject. Changes in blood pressure were only observed when a high dose (3 mg/kg) was used. The effects of this dose on performance depended on the impulsivity of the subject, with high impulsives performing worse in the de-caffeinated condition but getting a greater benefit from the caffeine. The high dose of caffeine also removed the post-lunch dip in sustained attention. The second experiment, which used a lower dose of caffeine (~60 mg), failed to demonstrate any caffeine x impulsivity or caffeine x time of day effects on performance. However, caffeine improved performance on a logical reasoning task and caffeine x time of day x impulsivity effects were found in analyses of visual search tasks. The mood data also support the view that the effects of caffeine depend on a combination of factors similar to those outlined for performance.