Abstract Introduction Daytime sleepiness is among the most frequent self-reported complaints by individuals who have sustained a mild traumatic brain injury (mTBI). Previous research demonstrates reduced vigilance and processing speed following mTBI. It has yet to be determined, however, if sustaining a mTBI alone, or the combination of daytime sleepiness and brain injury more greatly impacts cognitive function. The goal of this preliminary analysis was to determine the association between vigilance, daytime sleepiness, and mTBI. Methods A total of 137 adults (Mage = 24.89±7.2; 83 females) participated in the study, including 33 healthy controls (HCs) and 104 individuals with a documented mTBI within the preceding 12 months. Daytime sleepiness was assessed using the Epworth Sleepiness Scale (ESS), while daytime vigilance was measured using the Psychomotor Vigilance Task (PVT). To assess the effect of mTBI and daytime sleepiness on vigilance, we fit a Poisson regression to the number of lapses on the PVT, with group and ESS scores as predictors. Results ESS scores were significantly higher (p<.001) and there were significantly more PVT lapses (p=.03) in those with a recent mTBI, compared to HCs. For those with mTBI, the rate of lapses increased by 7.5% for every 1-point increase in ESS score (p< .001). Furthermore, when compared to HCs, the PVT lapse rate was 1.8x higher for individuals with a history of mTBI (p< 0.001), after controlling for ESS scores. Conclusion Daytime sleepiness was negatively associated with sustained vigilance for all participants. However, the magnitude of this association was roughly twice as high in individuals who had sustained a mTBI in the previous year. These findings provide evidence of a significant compounding effect of daytime sleepiness and brain injury on sustained vigilant attention. Clinical evaluation of mTBI would benefit from routine assessment of daytime sleepiness. Support USAMRMC grant (W81XWH-12–0386).