Multisensory perceptual decision-making persists as alertness fades, even in the absence of an overt response

Abstract Momentary declines in alertness are common in everyday life, yet we must continue to make rapid and accurate decisions from multiple sensory signals that may converge or conflict. Here we investigated multisensory perceptual decision-making across the wake-sleep transition. Participants performed an audio–tactile spatial localisation task while electroencephalography tracked spontaneous fluctuations in alertness. Although drowsiness slowed responses and increased omissions, participants remained largely accurate, and the behavioural influence of congruent and conflicting tactile information persisted. Behavioural modelling revealed slower evidence accumulation and increased decision boundaries. Stimulus- and response-related neural patterns remained robustly decodable across alertness states, despite changes in their temporal dynamics but multisensory congruency was no longer reliably decoded during drowsiness. Brain signatures of lateralized motor preparation persisted during behavioural omissions and, under multisensory conflict, might even favour the competing response. Declining alertness does not uniformly degrade multisensory perceptual decision-making but selectively alters the processes linking sensory information to action.