Effects of caffeine withdrawal on motor performance and heart rate changes

Heavy caffeine Users and Non-users were tested twice using a signalled reaction time paradigm, while heart rate was recorded. During the 7-day interval between the two sessions, the User group maintained normal caffeine intake for the first 5 days and abstained from caffeine during the last 2 days, which constituted a withdrawal period. The Non-users abstained from caffeine (as they usually do) during the entire 7-day period. The analyses of the reaction time (RT) data indicated no differences during Session 1 and a significant group difference during Session 2, with the Non-users exhibiting faster RT's. The Users exhibited no change between Sessions 1 and 2, while the Non-users showed a decrease in RT's (increased speed) across Sessions, which may be indicative of a practice effect. The analysis of the heart rate data from the 5-s period prior to the imperative stimulus yielded no differences during Session 1, and a significant group difference in Session 2, with the Non-users exhibiting larger decelerations. Across the two Sessions, the Non-users showed an increase in the magnitude of the heart rate deceleration, while the Users exhibited a decline in the size of the deceleration. Since the magnitude of heart rate deceleration has been interpreted as an index of attention, it appears that the Users were exhibiting less efficient attention during withdrawal from caffeine (Session 2) than were the Non-users. This conclusion is supported by the RT data, showing slower response times for the Users during Session 2, as compared to Non-users.