## 4 Discussion
### Discussion on Cumulative Effects of Long-Duration Driving  
Our analysis revealed significant cumulative effects in physiological signals over extended periods of driving, particularly evident in manual driving modes. For instance, under manual driving, the head heading gradually converged to 0 degrees, accompanied by a gradual decrease in the electrocardiographic index N50. In long driving tasks, the characterization of the driver's head heading on vigilance were significantly affected by driving time, while N50 was not significantly affected by driving time. The cumulative effect of head heading in manual driving may indicate a change in the driver's allocation of attention over time [50]. During extended periods of driving, drivers need to continuously monitor the road and surroundings, requiring frequent head turns to view mirrors and blind spots. Over time, they may adjust their head-turning strategy to allocate attention more efficiently. These adjustments may be reflected in the cumulative effect of head heading. The cumulative effect of head heading was present in manual driving mode, but not in automated driving mode. According to attention resource theory [51], this reflects the different effects of driving modes on driver behavior and attention demand. Manual driving requires the driver to make decisions and maneuvers, necessitating head turns for safety and accuracy. In contrast, automated driving systems take over most driving tasks, reducing the need for the driver to maneuver. Consequently, drivers in automated mode do not need to turn their heads as often for environmental monitoring or operational decisions. The decrease in N50 in manual driving mode indicates an increased workload resulting from extended driving time [52]. This cumulative effect may be associated with decreased heart rate variability, suggesting a limited ability of the heart to cope with sustained load [53]. In contrast, the stabilized ECG metrics in automated driving reflect an automated system that reduces the operational burden on the driver, thereby decreasing the workload. The cumulative effect of N50 appears in manual driving mode but not in automated mode. According to workload theory, this may be due to differences in driver workload between the modes. Manual driving requires continuous monitoring, decision-making, and operation, leading to a high cognitive and operational load that increases overall workload. In response, sympathetic nervous system activity increases while parasympathetic activity decreases [31]. Over time, this constant high-stress state may lead to a decrease in autonomic nervous system regulation, reflected by a significant decrease in N50. In automated mode, the system handles most driving tasks, significantly reducing the driver's operational loads. Due to lower stress, the driver's autonomic nervous system maintains a relatively stable state, resulting in no significant change in N50. In a long-duration driving task, the characterization of vigilance by the driver's head heading was significantly affected by driving time, and N50 was not significantly affected by driving time. The difference may be due to physiological versus behavioral differences: head heading is a direct indicator of driver behavior and attention allocation, whereas N50 reflects more the physiological state of the autonomic nervous system. The cumulative effect of prolonged driving on behavior is more pronounced because drivers need to constantly adjust their behavior to road and traffic conditions, whereas physiological metrics may show greater stability. The absence of cumulative effects in drivers' EEG metrics in manual or automated driving may reflect the brain's adaptation and regulation mechanisms. To cope with driving demands, the brain maintains the balance and stability of EEG activities through dynamic allocation of cognitive resources, avoiding cumulative effects. However, this contrasts with the cumulative effects found in the Theta and Alpha bands at Cz electrode sites in the literature [33].