Exposure to light at night (LAN) in residential environments has been suggested to be associated with a range of health outcomes, including sleep problems. Yet, the available evidence remains scarce. The study was based on data from 8284 adults (aged 35-70 years) residing in Yazd province, Iran, and participating in the Shahedieh Cohort Study enrolment phase (2015-2017). Sleep-related outcomes and covariates were collected through face-to-face interviews. Exposure to outdoor LAN (nanoWatts/cm2/sr) at different exposure windows was measured at residential addresses via satellite data. The association of LAN with sleep habits was tested using logistic regression models adjusted for relevant variables (reported as odds ratio [OR], and 95 % confidence intervals [CI], per interquartile range [IQR] increase of LAN). The outdoor LAN was in the range of 0.54-69.27 nanoWatts/cm2/sr (IQR: 14.91). A significant association was found between exposure to outdoor LAN and shorter sleep duration (OR: 1.21; 95 % CI: 1.10-1.32), and use of sleep medications (OR: 1.29; 95 % CI: 1.08-1.53). In contrast, greater LAN exposure was associated with lower odds for prolonged sleep latency. Associations were mostly stronger in women (e.g., use of sleep medications in men OR: 1.19; 95 % CI: 0.91-1.55 compared to women OR: 1.40; 95 % CI: 1.11-1.77), non-employed, and overweight/obese participants. Considering the large proportion of the exposed population in urban areas and the ongoing urbanization, outdoor LAN can be regarded as an increasing threat for public health.
