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Most studies in children using questionnaires or home polygraphy indicate that exposure to environmental tobacco smoke (ETS) is a significant risk factor for insomnia, snoring, and sleep-disordered breathing (SDB); however, objective polysomnographic evidence in suspected SDB is limited. We prospectively assessed ETS-sleep associations using urinary cotinine and overnight polysomnography in 30 children (mean age 5.8 ± 2.4 years; range 1-12; 73% boys) referred for SDB. Parents completed validated questionnaires on sleep and household ETS, and an Enzyme-Linked Immunosorbent Assay measured urinary cotinine. 46% had at least one smoking parent. Cotinine correlated with reported ETS exposure (r = 0.55, p = 0.03), supporting questionnaire validity. ETS exposure was not associated with apnea-hypopnea or oxygen desaturation indices, but exposed children had a 67% higher arousal index (p = 0.03), reflecting greater sleep fragmentation. Higher cotinine levels correlated with shorter sleep duration (r = - 0.36, p = 0.02) and lower sleep efficiency (r = - 0.43, p = 0.02). These findings indicate that ETS adversely affects sleep independent of respiratory events, highlighting a modifiable risk with measurable biomarker-linked impacts on pediatric sleep. Reducing ETS exposure may improve sleep quality even in children with SDB.