Drowsiness alters the neural dynamics but not the core computations of multisensory integration

Fluctuations in alertness shape perception and behaviour, yet how they affect the brains ability to integrate information across senses remains poorly understood. Here we investigated whether multisensory integration is preserved as humans transition from wakefulness to drowsiness. Participants performed an audio-tactile detection task while electroencephalography tracked spontaneous declines in alertness. Behaviourally, multisensory stimulation continued to facilitate responses during drowsiness, despite slowing and increased omissions. However, reaction-time distributions revealed reduced violations of race-model predictions, indicating weakened computational evidence for multisensory integration. Neurally, event-related signatures of multisensory interactions were delayed and attenuated during drowsiness. By contrast, multivariate decoding showed that multisensory neural representations remained detectable, although less temporally stable. Crucially, early cross-state generalization revealed shared representational structure across alertness states, indicating preserved core multisensory computations. These findings show that multisensory integration remains functional but is dynamically altered as alertness declines, revealing how the brain maintains adaptive behaviour during the wake-to-sleep transition.