Heart rate variability reveals graded task difficulty effects and sensitization dynamics in anticipatory psychological stress via time-domain analysis

Background Mental arithmetic tasks effectively induce psychological stress responses, but anticipatory stress responses before task onset are often overlooked. This study investigates how task difficulty influences anticipatory stress through heart rate variability time-domain analysis. Methods This study developed a standardized mental arithmetic task program using Unity, incorporating low, medium, and high levels by adjusting the amount of calculation and time limits. The participants were 12 healthy graduate and doctoral students. During the experiment, heart rate variability time indicators and the average RR interval were used as key physiological indicators to quantify psychological stress response. After the experiment, the participants were asked to complete the NASA Task Load Index (NASA-TLX) questionnaire to assess their workload. Results The NASA-TLX scores revealed significant differences in perceived workload among the three levels of task difficulty. The results indicated that task difficulty had a significant impact on anticipatory psychological stress response. High-level tasks elicited significantly greater anticipatory psychological stress responses compared to low-level tasks. Among the indicators used, the standard deviation of normal-to-normal (SDNN) intervals demonstrated particularly strong performance and may serve as a reliable and sensitive measure of anticipatory psychological stress response. Conclusions This study provides preliminary evidence supporting the use of SDNN as a complementary physiological indicator of anticipatory psychological stress responses. The findings suggest that task difficulty not only modulates individuals’ anticipatory psychological responses on a cognitive level but also significantly shapes the dynamic trajectory of the SDNN during stress development. The observed sensitization effect indicates that higher-difficulty tasks can lead to enhanced anticipatory psychological stress responses in subsequent tasks. These results have potential implications for optimizing psychological stress response intervention strategies and for the development of standardized and replicable paradigms for anticipatory psychological stress research. Future studies should incorporate a larger and more diverse sample to further investigate how individual differences influence anticipatory psychological stress responses.