Down's syndrome (DS) predisposes to sleep-disordered breathing (SDB). In children with DS, behavioural and emotional disturbances secondary to SDB are often assumed to result from cognitive impairment alone. Our aim was to explore the relationship of behavioural and emotional disturbances with SDB in a population of children with DS. A modified sleep questionnaire, Epworth Sleepiness Scale (ESS), Paediatric ESS and the short form of the developmental behaviour checklist (DBC-P24) were sent to 261 carers of children aged 4 to 15 years with DS in 2012. Of 120 participants, 25% had probable SDB. In children with probable SDB compared to those without nocturnal symptoms, the total behaviour problem score (TBPS) was significantly higher (20.3 ± 8.6/48 vs. 12 ± 7.5/48; P = 0.002) as was the PaedESS (7.7 ± 5.6/24 vs. 2.8 ± 3.5/24; P = 0.002). For every increase in frequency of choking attacks, snoring and night awakenings, the TBPS increased by 1.37, 1.28 and 1.75 points, respectively, indicating worsening behaviour. The TBPS was found to decrease by 1.31 points for every hour more of self-reported sleep duration (r = -0.25, P = 0.017). SDB symptoms and shorter self-reported sleep duration are highly prevalent among children with DS and are independently associated with worsening behaviour using the TBPS. © 2020 MENCAP and International Association of the Scientific Study of Intellectual and Developmental Disabilities and John Wiley & Sons Ltd.