Circadian-dependent neural mechanisms of lighting optimization in underground transit environments: Evidence from...

Underground metro cabins rely almost entirely on artificial lighting, which simultaneously provides visual input and circadian cues for passengers. Yet, neurophysiological evidence remains scarce on how time-of-day-specific illuminance and correlated color temperature (CCT) jointly shape functional brain organization under metro lighting conditions. We applied a 3 (time-of-day illuminance) × 3 (CCT) mixed design to simulate metro cabin lighting and tested 60 healthy adults across morning, afternoon, and evening sessions. Prefrontal hemodynamics were recorded using functional near-infrared spectroscopy (fNIRS). Region of interest functional connectivity was quantified with Network-Based Statistic (NBS) and graph metrics. In the afternoon under 4300 K lighting, the dorsolateral prefrontal cortex (DLPFC) exhibited the most efficient network integration, reflected by the minimum characteristic path length (Lp). Mechanistically, cool light (6500 K) enhanced coupling between the ventromedial prefrontal cortex (vmPFC) and the premotor cortex (PMC), which was associated with increased alertness. Warm light (2700 K) was related to improved mood regulation, whereas neutral light (4300 K) optimized cognitive processing. Key hubs including the vmPFC, DLPFC, and PMC, linked light exposure to emotion, executive, and motor networks. These findings provide neurophysiological evidence for circadian aligned metro lighting strategies that may support passenger well-being in enclosed transit environments.