This research aimed to evaluate an existing sleepiness detection warning system for drivers by collecting the heart rate measurement (beat per minute) of 12 participants while driving a driving simulator. During conducting research, the participants were asked to drive a driving simulation for 30 min per four sessions. Additionally, in the fourth session, the participants were equipped with a warning system that was designed to detect sleepiness. The sleepiness behaviour of the driver was also observed and compared to the modified Karolinska Sleepiness Scale (KSS). Next, the data collected were analysed using the Repeated Measures ANOVA. The result showed that the overall mean heart rate of the participants tended to be the highest mean value at the first session and gradually decreased until the third session. Meanwhile, in the fourth session, the mean beat per minute (bpm) of heart rate increased drastically. Based on findings, it showed that the warning device successfully detected the difference in the heart rate of the driver. Additionally, the behaviour depicted when the driver’s sleepiness was evaluated based on the Driver’s Behaviour Sleepiness Scale (DBSS) and compared to KSS. Overall, the research suggested that it can prove the effectiveness of the existing warning system and contribute to enhancing road safety.