Sleep health impact and its projected productivity loss of secondhand smoke exposure among non-current smokers in China: an analysis of a 0.5-million prospective cohort

Background: Prospective evidence on secondhand smoke (SHS) exposure and sleep health in adults is lacking, and the potential economic impact has not been quantified. We examined associations between SHS and sleep outcomes in a large Chinese cohort. Methods: From 512,724 adults aged 30–79 in the China Kadoorie Biobank (2004–2008), we analysed 346,222 non-smoking participants after excluding current smokers. Frequent SHS (≥1 day/week) was the primary exposure. Four sleep outcomes were assessed. Entropy-balanced logistic regression estimated odds ratios (ORs). Prospective analyses used data from resurvey (2013–2014; N=16,790). Changes of exposure status, different exposure settings, dose-response, and related productivity loss analyses were also conducted. Findings: Frequent SHS was associated with insufficient sleep (OR 1·26, 95% CI 1·22–1·29), early morning awakening (1·10, 1·07–1·13), and functional impairment (1·23, 1·17–1·29). Prospectively, baseline SHS predicted early morning awakening (1·24, 1·14–1·34) and functional impairment (1·23, 1·11–1·36). Persistently exposed participants had the highest odds (early morning awakening 1·53, 1·35–1·74), with a gradient consistent with a causal relationship. Household SHS showed the strongest association with difficulty falling asleep (1·47, 1·32–1·64). Under a scenario analysis assuming causality, approximately 12% of functional impairment was attributable to SHS, with projected annual productivity losses of 15·1 billion yuan (range 1·2–34·6 billion). Interpretation: Frequent SHS exposure is associated with poorer sleep health in non-smoking adults, with prospective and exposure-change evidence consistent with a causal relationship. A scenario analysis projected annual productivity losses of 15·1 billion yuan. China lacks a comprehensive national smoke-free law, and enforcement in public and workplace settings remains uneven. Strengthening smoke-free protections across public venues, workplaces, and homes could yield co-benefits for sleep health and productivity.