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Abstract

CAFFEINE AND RATINGS OF ALERTNESS IN THE LATE MORNING

Andrew P. Smith PhD*

ABSTRACT

Background: Subjective alertness reaches a peak in the late morning. Previous research demonstrates that caffeine increases alertness when given in the early morning. This supports the view that caffeine is beneficial in low alertness situations. If caffeine mainly restores function when it is below optimum, one would expect less effect when circadian alertness is high in the late morning. This study examined this issue and whether any effects could be attributed to the reversal of caffeine withdrawal. Methods: Three groups of participants were recruited: non-consumers of caffeine (N=23); low consumers (<200md/day; N=40) and high consumers (>300mg/day; N=33). After overnight caffeine abstinence, participants visited the laboratory and rated their alertness at 09.00. They then consumed a fruit juice which had either caffeine (100 mg) or placebo added. Two hours later, they returned to the laboratory and drank a fruit tea which again contained either 100 mg caffeine or the placebo. They then returned to the laboratory at 13.00 and rated their alertness. They repeated this procedure for five consecutive days. Results: Caffeine was associated with higher alertness, but this effect was not significant. This effect did not change over days, and the absence of a significant effect of caffeine was observed in consumers and non-consumers. Conclusion: These results demonstrate that caffeine had no significant effect on alertness ratings in the late morning. This was reliable in that it did not change across days or with consumer status. These results support the view that caffeine has its clearest effects when alertness is below optimum. Alertness is below optimum for a large part of the day and can also be reduced by changes to the sleep-wake cycle and exogenous factors which increase fatigue. Previous results have important implications for real-life situations involving low arousal states and show that caffeine is an effective countermeasure when circadian alertness is low, whereas the present findings confirm that it has less effect when alertness is near its peak.

Keywords: Caffeine; Caffeine Withdrawal; Circadian Arousal; Ratings of Alertness.


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