Nightlight-Nightlife Interplay: The threshold effect of ambient lighting on perceived safety and physical activity at night

• First city-wide high-fidelity nighttime SVIs generated for Hong Kong. • Image-based brightness validated against field illuminance measurements. • Eye-level brightness outperforms satellite radiance to explain nighttime exercise. • Near-field and light-of-sight lighting facilitate perceived safety. • Perceived safety gains diminish beyond a 40–60 Luma lighting threshold. Artificial light at night (ALAN) influences safety perceptions, public health, energy use, and biodiversity, yet its eye-level impacts remain difficult to quantify at city scale. Meanwhile, reducing outdoor lighting is especially contested because the widely held belief that “brighter is safer” sustains excessive ALAN even in low-crime cities such as Hong Kong. We assert that unveiling the threshold effect of brightness on perceived safety and human behavior is crucial for shifting the linear paradigm and informing acceptable lighting adjustments. However, auditing urban-scale nighttime perceived safety is challenging because nighttime street view imagery (SVI) is almost nonexistent. Most studies rely on limited nighttime samples, satellite radiance, or daytime SVI as proxies, ignoring the spatial heterogeneity of nightscapes and differences between day and night. Moreover, lighting intensity is often examined in silo; how it interacts with urban forms is underexplored. To address these gaps, we developed a multimodal diffusion model trained on 2600 paired day–night SVIs and conditioned on SDGSAT-1 radiance (40 m) and POI context to synthesize context-aware nighttime scenes from 58,500 daytime SVIs (50 m spacing) across Hong Kong. A validated vision-language model was used to score perceived nighttime safety from generated images, while semantic area, brightness, and depth (ABD) features were extracted to model their interactions. To characterize human behaviors, approximately 150,000 volunteered trajectories of physical activities were collected. The ABD interactions extracted from eye-level nightscapes alone explained 49.3% (36.5%) of the variation in day–night activity disparities (perceived nighttime safety), significantly outperforming satellite-based radiance. Near-field, line-of-sight lighting on facades, trees, and sidewalks is consistently associated with higher safety and stronger nighttime activity retention. Brightness shows diminishing gains beyond ∼ 40–60 luma DN (∼19–40 lx as an interpretive reference from our DN–lux field cross-walk) on nighttime perception and physical activity across heterogeneous urban contexts, indicating significant potential for reduced light pollution in overlit areas. This study demonstrates the necessity and provides a scalable toolkit for auditing nighttime environments at eye level to support sustainable and healthy cities.