CAFFEINE, MEMORY, IMPULSIVITY AND TIME OF DAY

Dominic P Nguyen-Van-Tam PhD and Andrew P Smith PhD*

ABSTRACT

Background: Despite the substantial research on the effects of caffeine on behaviour, there have been relatively few investigations of individual differences and the impact of time of day. Aims: The present study tested the model of Humphreys and Revelle (1984) which regards caffeine as a source of arousal which interacts with baseline physiological arousal, as measured by the personality trait of impulsivity and time of testing. Methods: The experimental design included the between-subject factors of impulsivity (high/low), time of day (morning/evening) and caffeine (4mg/kg caffeine/placebo). Ninety-six participants completed the study. Testing was carried out on two consecutive days and the participants rated their mood and performed semantic processing, logical reasoning and levels of processing memory tasks. Results: The results showed that caffeine increased alertness and improved performance on sematic processing and logical reasoning tasks. Performance of these tasks was also influenced by time of testing. Low impulsive participants reported higher alertness in the morning compared to the evening, whereas high impulsive participants showed the opposite profile. There was little evidence of interactions between caffeine and impulsivity, or caffeine x impulsivity x time of day. Conclusions: The present study showed that alertness, logical reasoning and semantic processing change after caffeine ingestion. Time of day and impusivity also influence some of these outcomes. However, the effects appear to be largely independent and there was little evidence of interactions between caffeine, impulsivity and time of day.

Keywords: Caffeine, Memory, Semantic processing, Logical reasoning, Alertness, Impulsivity, Time of day.