Heat stress and sleep quality in Mediterranean cities: a pilot study introducing the Sleep Comfort Index (SCI)

Abstract Urban heat increasingly affects sleep quality, yet quantitative evidence linking outdoor thermal stress and rest remains limited. This pilot study, conducted in Seville during the summer of 2023, examined how outdoor temperatures influence perceived sleep comfort. A total of 237 in situ outdoor thermal-comfort surveys (22.6 °C–41.1 °C; 14.7%–54.7% RH) were used to characterise daytime thermal perception and identify local comfort thresholds, and were combined with ERA5-derived solar radiation and wind speed data, as well as an online sleep questionnaire (n = 252). Most participants (≈ 80%) reported not using air conditioning at night, and temperature was widely recognised as a key factor affecting sleep. Nights with temperatures above 30 °C coincided with a marked decrease in reports of sufficient sleep (53.9% on cooler nights versus 39.1% on hotter nights). Logistic regression confirmed this trend: each additional degree of night-time temperature reduced the likelihood of reporting sufficient sleep by about 4% (OR = 0.96; pseudo-R 2 = 0.012). Based on these observations, the Sleep Comfort Index (SCI) was developed to reflect the combined influence of daily and nightly thermal exposure on sleep quality. The index integrates both periods using weighted penalties (w_d = 0.41; w_n = 0.59), giving greater importance to night-time conditions. SCI scores approached 5 when daytime temperatures ranged from 24 °C–30 °C and night-time values hovered near 26 °C, while values dropped below 3 when night temperatures exceeded 30 °C. Notably, sleep-related temperature thresholds closely overlapped with the upper limits of outdoor thermal comfort identified in the in situ survey, supporting a thermal-stress interpretation of sleep deterioration. Designed for flexibility, the SCI can be recalibrated for other climates by adjusting preferred temperature ranges and weighting coefficients, offering a practical tool for exploring links between outdoor thermal conditions, behavioural adaptation, and sleep-related well-being in hot Mediterranean settings.