To alleviate the workload of long-haul truck drivers and subsequently reduce their fatigue, companies have begun exploring the implementation of advanced driver assistance systems (ADAS) on heavy trucks to aid drivers with certain driving tasks. However, prior research conducted in the passenger car domain has suggested that drivers utilizing ADAS are more susceptible to experiencing fatigue in comparison to manual driving. Therefore, it is imperative to examine the impact of ADAS on the fatigue levels of truck drivers. Based on over 120 h of naturalistic driving experiments, this study employs various eye-tracking metrics (e.g. blink frequency), response time, and subjective ratings to compare the fatigue levels of long-haul heavy truck drivers when driving manually versus driving with the assistance of ADAS. The results indicate that the fatigue levels of long-haul truck drivers are lower when operating vehicles with the assistance of ADAS in comparison to manual driving.