Urban chronotypes as climate vulnerability indicators: Safety-modulated bioclimatic responses and adaptive behavior across diurnal urban contexts

Urban heat adaptation strategies assume stable human bioclimatic potential responses across time and space, yet cities transform substantially between day and night: thermally, acoustically, visually, and in terms of perceived safety, fundamentally altering how populations sense and respond to environmental exposures. Laboratory-based thermal comfort frameworks miss this real-world contextual complexity. This study introduces urban chronotypes: four distinct patterns of physiological and behavioral response to urban climatic conditions emerging from the interaction of time of day and perceived safety and examines their implications for climate vulnerability and heat adaptation effectiveness. In Brussels Capital Region during summer 2025, 36 monitoring sessions were conducted across morning and night walks through validated high-safety and low-safety urban areas, generating 2132 validated exposure-physiology-behavior pathway observations using synchronized wearable environmental, physiological, and eye-tracking sensors. Three findings emerge. First, unsafe contexts reversed physiological-behavioral coupling from positive to near-zero, demonstrating that cooling strategies deployed in unsafe areas fail to produce protective behavioral responses despite intact physiological heat sensing, a climate adaptation failure disproportionately affecting urban dwellers. Second, thermal bioclimatic processing showed unique context-dependence (+24% coupling in nighttime conditions, Bonferroni-corrected), while air quality, acoustic, and visual processing remained stable across all contexts. Third, nighttime processing accelerated by 3.6 min regardless of safety context, establishing a universal 14–16 min adaptation window as the minimum duration urban climate interventions must persist to enable complete physiological-behavioral integration. Urban climate vulnerability is not solely determined by thermal exposure magnitude but is critically shaped by perceived safety and temporal context, demanding integrated climate-safety approaches to urban heat adaptation. • Safety perception disrupts translation of thermal responses into adaptive behavior. • Unsafe areas create climate vulnerability invisible to exposure-only frameworks. • Thermal sensitivity increases 24% in safe nighttime contexts. • 14–16 min adaptation window identifies optimal intervention timing. • Four urban chronotypes reveal when and where cooling infrastructure works best.