A composite Sleep Thermal Health Index (STHI) framework for sleep-period thermal health under warm nights with PLS-SEM-based composite pathway analysis

Hot nights are becoming more frequent and undermine physiological recovery during sleep, yet building operation remains largely guided by wake-centric comfort proxies and fragmented sleep endpoints. This study develops the STHI as a formative composite index for summarizing sleep-period thermal burden and downstream recovery. A controlled within-subject sleep-laboratory protocol was conducted in 35 healthy adults under five constant ambient temperatures (22, 24, 26, 28, and 30 °C). Multimodal observations included multi-site skin temperature for thermophysical load computation, wrist actigraphy sleep metrics, wearable electrodermal activity and HRV features, in-sleep thermal sensation and comfort ratings, and next-day mood and cognitive performance. The STHI was constructed by harmonizing indicator directionality, mapping standardized signals to bounded scores, and hierarchically aggregating domains covering thermal experience, physiological regulation, sleep integrity, and next-day function. The index showed high agreement across alternative weighting schemes, supporting its use as a comparative assessment tool. At the composite level, STHI decreased nonlinearly with warming, with larger losses emerging above the lower-temperature range. Quadratic mixed-effects modelling supported this buffered-to-accelerating pattern across 22–30 °C. A within-subject PLS-SEM-based composite path analysis examined whether the observed block relationships were consistent with the hypothesized exposure–autonomic–sleep–function pathway; electrodermal arousal showed the strongest observed transmission pathway in this model. Exploratory sex-stratified analyses suggested a largely shared pathway architecture, with a possible difference in the sleep-integrity-to-cognition linkage. The proposed STHI provides an evaluation interface for comparing night-time temperature regimes and for informing future recovery-aware building-operation studies under increasingly warm indoor conditions.