Caffeine and time of day effects on a force discrimination task in humans

Caffeine and Exercise: What Next?

Caffeine is a widely utilized performance-enhancing supplement used by athletes and non-athletes alike. In recent years, a number of meta-analyses have demonstrated that caffeine's ergogenic effects on exercise performance are well-established and well-replicated, appearing consistent across a broad range of exercise modalities. As such, it is clear that caffeine is an ergogenic aid-but can we further explore the context of this ergogenic aid in order to better inform practice? We propose that future research should aim to better understand the nuances of caffeine use within sport and exercise. Here, we propose a number of areas for exploration within future caffeine research. These include an understanding of the effects of training status, habitual caffeine use, time of day, age, and sex on caffeine ergogenicity, as well as further insight into the modifying effects of genotype. We also propose that a better understanding of the wider, non-direct effects of caffeine on exercise, such as how it modifies sleep, anxiety, and post-exercise recovery, will ensure athletes can maximize the performance benefits of caffeine supplementation during both training and competition. Whilst not exhaustive, we hope that the questions provided within this manuscript will prompt researchers to explore areas with the potential to have a large impact on caffeine use in the future.

The effect of caffeine on cognitive task performance and motor fatigue

RATIONALE

In everyday life, people are usually capable of performing two tasks simultaneously. However, in a previous study we showed that during a fatiguing motor task, cognitive performance declined progressively. There is extensive literature on the (positive) effects of caffeine on cognitive and motor performance. These effects are most pronounced under suboptimal conditions, for example during fatigue. However, little is known about the effects of caffeine on cognitive performance during a fatiguing motor task.

OBJECTIVE

This study was aimed to investigate whether a moderate dose of caffeine could attenuate the decline in cognitive performance during a fatiguing motor task.

METHODS

The study consisted of a placebo and a caffeine (3 mg/kg) session. A total of 23 subjects completed these sessions in a semi-randomized and double-blind order. In each session, subjects performed maximal voluntary contractions of the index finger, a choice reaction time (CRT) task and a dual task consisting of a fatiguing motor task concomitantly with the same CRT task. After the fatiguing dual task, the CRT task was repeated.

RESULTS

Caffeine improved cognitive task performance, in both the single and dual task, as shown by decreased reaction times together with unchanged accuracy. Cognitive performance in the dual task deteriorated with increasing fatigue. However, the decrease in cognitive performance in the beginning of the dual task, as observed in the placebo condition, was partly prevented by caffeine administration (i.e., no increase in reaction times). We found no effects of caffeine on motor parameters (absolute force, endurance time or electromyographic amplitude).

CONCLUSIONS

Caffeine improved cognitive performance. This effect also extends under demanding situations, as was shown by the performance during the dual task, even during progressive motor fatigue.

Tolerance to daytime sedative effects of H1 antihistamines

Sedation is the principal side effect of first generation H1 antihistamines, and recent studies have suggested that this side effect should limit the clinical application of these drugs. The sedative effect also underlies the use of these first-generation drugs as nonprescriptive remedies for insomnia. In both cases, the potential for tolerance to the sedative effect of these drugs is an important issue for which there are few objective data. In the study reported here, 15 healthy men age 18 to 50 years received either diphenhydramine 50 mg or placebo twice a day for 4 days in a randomized, double-blind, crossover trial design. Dependent measures included objective and subjective assessments of sleepiness and computer-based tests of psychomotor performance. Both objective and subjective measures of sleepiness showed significantly higher levels on day 1 for diphenhydramine compared to placebo. By day 4, however, levels of sleepiness on diphenhydramine were indistinguishable from placebo. Similarly, diphenhydramine produced significant impairment of performance that was completely reversed by day 4. These data provide the first objective confirmation that tolerance develops to the sedative effect of a prototypical first-generation H1 antihistamine, diphenhydramine. On this dosing regimen, tolerance was complete by the end of 3 days of administration. While other antihistamines and dosing regimens may differ, these results suggest that tolerance to the sedation produced by these drugs develops with remarkable rapidity.

Regular caffeine consumption: a balance of adverse and beneficial effects for mood and psychomotor performance

It has often been pointed out that caffeine is the most widely "used" psychoactive substance in the world, and accordingly, there is a very large amount of research available on the effects of caffeine on body and mind. In particular, a psychostimulant action of caffeine is generally accepted as well established; for example, caffeine has been found to quicken reaction time and enhance vigilance performance, and to increase self-rated alertness and improve mood. There is, however, a real difficulty in determining the net effects of caffeine. In a typical experiment the subjects have a history of regular caffeine consumption, and they are tested on caffeine and a placebo after a period of caffeine deprivation (often overnight). The problem with relying solely on this approach is that it leaves open the question as to whether the results obtained are due to beneficial effects of caffeine or to deleterious effects of caffeine deprivation. The present article briefly reviews this evidence on the psychostimulant effects of caffeine, and presents some new data testing the hypothesis that caffeine may enhance cognitive performance to a greater extent in older adults than in young adults. No age-related differences in the effects of caffeine on psychomotor performance were found. We conclude that overall there is little unequivocal evidence to show that regular caffeine use is likely to substantially benefit mood or performance. Indeed, one of the significant factors motivating caffeine consumption appears to be "withdrawal relief."

Gender and impulsivity differences in licit substance use

Gender differences in ratings of impulsivity have shown increased impulsivity in men. Research also suggests higher impulsivity is correlated with increased use of alcohol, nicotine, and caffeine. In a large sample of young adults (N = 332), self-report data were collected on licit substance use and impulsivity/nonconformity. Among men, significant differences were found between higher and lower impulsivity groups for alcohol and caffeine use but not for nicotine use. Among women, significant differences were found between higher and lower impulsivity groups for alcohol and nicotine use but not for caffeine use. The data suggest the importance of recognizing gender differences in the relationship between personality factors and licit substance use.

Caffeine, but not time of day, increases whole-arm physiological tremor in non-smoking moderate users

1. Consumptive levels of caffeine significantly increased whole-arm physiological tremor in young adult males at 3 but not 1 mg caffeine/kg bodyweight. 2. Physiological tremor was not affected by time of day (TOD) or the interaction of caffeine and TOD. 3. Findings suggest that ingestion of a single oral dose of caffeine at typical consumptive levels results in measurable changes in physiological functioning and that whole-arm physiological tremor is a sensitive measure of physiological change due to drug effects. 4. However, the results suggest that whole-arm physiological tremor may not detect reliably TOD changes in functioning.

Caffeine enhances initial but not extended learning of a proprioceptive-based discrimination task in nonsmoking moderate users

We examined the effects of caffeine on fine motor performance and learning using a multiple-force discrimination task. 93 college-aged subjects performed this task on which multiple measurements were made in an operant response paradigm. Quantitative measures of accuracy of response, duration of response, latency of response, force, and variability of force were examined. Significant interactions for caffeine dose by session on accuracy of response and latency of response indicated that caffeine enhanced the initial learning of a proprioceptive motor task but did not improve performance beyond that of normal practice.