Functional Near-Infrared Spectroscopy Reveals Stable Visually-Evoked Cortical Responses across Circadian Phases

Circadian rhythms shape physiology and behavior. However, their influence on sensory-evoked neural activity remains poorly understood. Although visual perception fluctuates across the day, it is unclear whether these fluctuations reflect changes in early neural processing or downstream cognitive states. Here, we tested whether visually-evoked cortical responses measured with functional near-infrared spectroscopy (fNIRS) are modulated by circadian phase or by repeated measurements across time. Hemodynamic responses over occipital cortex were recorded in healthy adults during a flickering visual stimulation paradigm. Participants were assessed either (i) at three circadian timepoints within a single day and again on separate days at matched hours, or (ii) across short-, medium-, and long-term test–retest intervals, including five minutes, two months, and two years. We found that visually-evoked response amplitudes remain stable across circadian phases and longitudinal intervals. These results indicate that early visual cortical responses measured with fNIRS are largely resilient to both circadian modulation and repeated stimulation. The marked temporal stability of fNIRS signals supports their reliability for longitudinal sensory research and highlights the potential of fNIRS as a non-invasive biomarker of cortical function.