Two experiments examined individual differences in lapses of sustained attention. Participants performed variants of the psychomotor vigilance task while pupillary responses and fixations were recorded. Examining pupillary responses during the interstimulus interval in both experiments suggested that individuals particularly susceptible to lapses of attention (indexed by the slowest response times) demonstrated a decreased pupillary response during the interstimulus interval, whereas individuals less susceptible to lapses of attention demonstrated an increased pupillary response during the interstimulus interval. These results suggest that variation in lapses of attention are partially attributable to individual differences in the ability to voluntarily control the intensity of attention (intrinsic alertness) and fully engage preparatory processes on a moment-by-moment basis. Furthermore, across both experiments additional individual differences factors covaried with lapses of attention, including attention control, working memory capacity, susceptibility to off-task thinking, task-specific motivation, and fixation stability. These results provide evidence for the notion that individual differences in lapses of attention are multifaceted and that variation in intrinsic alertness and other factors are important contributors to this variation. (PsycINFO Database Record (c) 2020 APA, all rights reserved).