Recent evidence highlights the potential impact of outdoor artificial light at night (ALAN) on sleep, and later sleep timing may be a risk factor for executive dysfunction. Depression is commonly associated with executive dysfunction, which significantly compromise prognosis. However, it remains unclear whether sleep-wake phase mediate the effects of outdoor ALAN on executive function. We assessed whether sleep timing mediated the association between outdoor ALAN and executive function among depressed patients. Between 2017 and 2023, 798 inpatients with depression were enrolled from the Fourth People's Hospital of Hefei. The outdoor ALAN exposure of patients was estimated using satellite images. All participants were assessed for sleep conditions using the Pittsburgh Sleep Quality Index (PSQI) and for executive function using the Behavior Rating Inventory of Executive Function-Adult Version (BRIEF-A). We employed multifactor linear regression models to investigate the association between outdoor ALAN exposure and executive function, adjusted for potential confounders. Additionally, mediation models were utilized to explore the mediating role of sleep timing in this association. Our study found that greater ALAN exposure was significantly associated with higher executive function scores. The exposure of participants at 12 months prior to enrollment had the greatest effect, for each interquartile range (IQR 29.93 nW/cm/sr) increase in outdoor ALAN exposure, the Global Executive Composite (GEC) scores increased by 6.12 (95 % confidence interval [CI]: 2.49, 9.74). Mediation analysis indicated that sleep midpoint significantly mediated the relationship between outdoor ALAN and executive function, with a mediated proportion of 22.39 %. The study suggested that outdoor ALAN exposure has adverse effects on executive function in depressed patients, and sleep-wake phase may mediate the association between outdoor ALAN exposure and executive function. 2
