In 2018, the World Health Organization reported about 1.35 million deaths caused by traffic accidents, which are also the leading cause of death of children and adolescents globally. Therefore, an early-warning system is needed to prevent accidents caused by fatigue driving. Research on driving-fatigue detection has determined the ground truth of the fatigue state by designing tests for specific conditions, designing tests at particular times, and relying on subjective reporting. Because determining the ground truth of the fatigue state affects the results of fatigue detection, this paper advances this investigation of the ground truth of the fatigue driving state and driving times using electrocardiogram recordings and a questionnaire that subjectively captures drivers' fatigue states. After five test sessions and heart-rate statistical data analysis, the minimum time required to induce a fatigue driving state in a driving simulation was 90 minutes with a Chalder Fatigue Scale score of 16. The driving-simulation software also affected the drivers' heart rates, however, so better programmable driving-simulation software is needed to create specific conditions, such as traffic density and limited driving speed, to induce realistic fatigue driving states.