Effects of repeated doses of caffeine on mood and performance of alert and fatigued volunteers

Evidence for behavioural effects of caffeine is well documented in the literature. It is associated with increased subjective alertness, improved reaction time and enhanced encoding of new information. These effects are most prominent in low arousal situations. However, there is an ongoing debate as to whether such changes are in fact improvements or merely a reversal of the negative effects of a period of caffeine withdrawal (e.g. overnight abstinence). To avoid such a confound this study included multiple doses of caffeine which were administered under double-blind conditions to participants who had ingested their normal daily quota of caffeine. In the present study participants were fatigued by carrying out a prolonged testing schedule in the evening. Sixty volunteers, all regular caffeine consumers, took part in the study. They attended for three sessions on separate days. They were instructed to consume normal amounts of caffeinated beverages. Consumption was measured by a diary and saliva samples were taken and caffeine assays conducted. A baseline test session was carried out at 18.00h and following this a double blind placebo controlled caffeine challenge (1.5mg/kg) conducted. The test battery was repeated twice approximately 30 minutes after the caffeine challenge. Following this another drink was administered and the test battery repeated twice more. On one test session volunteers had placebo in both drinks, in another they had caffeine in both drinks and another caffeine in the first and placebo in the second. Order of conditions was balanced across subjects. The results showed that caffeine led to a more positive mood and improved performance on a number of tasks. Different effects of caffeine were seen depending on the person's level of arousal. Linear effects of caffeine dose were also observed. This is evidence against the argument that behavioural changes due to caffeine are merely the reversal of negative effects of a long period of caffeine abstinence. The findings are discussed in relation to both noradrenergic and cholinergic neurotransmitter systems.