Adaptive behavior often requires rapidly and flexibly shifting focus in response to unexpected events. Such spatial attention may depend on fluctuations in arousal, a relationship that remains poorly understood. This study examined how arousal fluctuations, indexed by pre- stimulus pupil size, influence spatial attention during an exogenous Posner cueing task. Guided by the Yerkes-Dodson law (YDL), we hypothesized a U-shaped relationship between arousal levels and the cueing effect (CE), predicting that both low and high arousal would impair attentional reorienting, while moderate arousal would enhance it. Additionally, we investigated whether caffeine modulates this relationship, hypothesizing a caffeine-arousal interaction that would shift the U-shaped arousal-spatial attention curve rightward, indicating better spatial reorienting at higher arousal levels among caffeine consumers. Thirty-one young adults completed the task while pupil size was continuously recorded using eye-tracking. Linear mixed-effects modeling assessed the relationship between arousal fluctuations and CE, as well as the potential modulating effect of caffeine. Contrary to our expectations, neither a quadratic nor a linear model consistently explained the relationship between pre-stimulus arousal and CE. Although caffeine appeared to interact with the quadratic arousal term, follow-up analyses within caffeine and no-caffeine groups did not confirm a significant pattern. These findings challenge the generalizability of the YDL to spatial attention and suggest that caffeine’s influence may be more complex. However, important to note is that methodological limitations may have obscured potential relationships. Consequently, while our results did not confirm the predicted U-shaped relationship, this does not necessarily rule out its existence.