Cardiac reactivity during task performance: influence of time of day

We evaluated the effect of time of day on cardiac reactivity (CR) during the performance of two vigilance tasks (auditory and visual reaction 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 women. Overall, the vigilance and memory tasks decreased and increased heart rate, respectively, and in the vigilance tasks this effect was dependent 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 deceleration to acceleration) in the visual vigilance task. We conclude that time of day has a significant effect on CR and that studies should take this factor into account. Authors should refrain from generalizing from results recorded at a single time of day.