["Psychophysiology, Volume 63, Issue 5, May 2026. ", "\nABSTRACT\nHypervigilance usually refers to the enhancement of external attention in an effort to scan and detect potential threats in the environment. Yet, whether this affective state could also influence internal attention or the ability to switch between external and internal attention (i.e., attentional balance) remains unclear. Recent studies have shown that attentional balance is characterized by internal dominance, raising the intriguing possibility that hypervigilance might alter attentional balance rather than merely boosting external attention. To test this hypothesis, we recorded behavioral, EEG, and eye‐tracking data from 46 participants performing the Switching Attention Task (SAT) under hypervigilance versus two other control conditions. The SAT has previously been validated to explore attentional balance. Hypervigilance was induced during the SAT through a task‐unrelated aversive sound played unpredictably, and its induction was confirmed by skin conductance, subjective ratings, and eye fixation data. At the behavioral level, using a drift diffusion model, we found that hypervigilance impaired attentional balance during the SAT, by decreasing the shielding of internal attention. Interestingly, we found that this impairment was captured by an amplitude modulation of the P2 event‐related potential. A joint modeling approach further confirmed this specific brain‐behavior association. Together, these findings suggest that hypervigilance modulates attentional balance through a selective modulation of internal attention.\n"]