CARDIAC REACTIVITY DURING TASK-PERFORMANCE - INFLUENCE OF TIME OF DAY

WE evaluated the effect of time of day on cardiac reactivity (CR) duri ng the performance of two vigilance tasks (auditory and visual reactio n time) and a working memory task. Heart rate was measured hourly (08: 00-21:00 h) at rest and during performance tasks in 20 men and 20 wome n. Overall, the vigilance and memory tasks decreased and increased hea rt rate, respectively, and in the vigilance tasks this effect was depe ndent on both time of day and sensory modality. The critical periods, those of greater stress, were the earliest and latest hours of the day and after lunch. Time of day also reversed the sign of CR (from decel eration to acceleration) in the visual vigilance task. We conclude tha t time of day has a significant effect on CR and that studies should t ake this factor into account. Authors should refrain from generalizing from results recorded at a single time of day.