Accelerometer-based 24-hour movement behaviours in children and adolescents with Down Syndrome Running title: 24-Hour Movement Behaviours in Down Syndrome

<title>Abstract</title> <p> Purpose Children and adolescents with Down syndrome (DS) may show altered 24-hour movement behaviours. This study compared physical activity, sedentary behaviour, sleep and rest–activity rhythms in paediatric DS with those of peers with normal weight and obesity. Methods This cross-sectional study included 51 participants aged 8–18 years: 17 with DS, 17 with normal weight and 17 with obesity. Seven-day accelerometry assessed activity, sedentary behaviour, sleep and rest–activity rhythms. Group differences were analysed using adjusted ANCOVA, PERMANOVA and principal component analysis. Results Compared with normal-weight peers, participants with DS had significantly greater sedentary time and lower light, moderate and moderate-to-vigorous physical activity (all <italic>P</italic>  &lt; 0.001). They also exhibited longer sedentary bouts ( <italic>P</italic>  &lt; 0.001), shorter physical-activity bouts ( <italic>P</italic>  = 0.037), a lower probability of transitioning from sedentary time to physical activity ( <italic>P</italic>  &lt; 0.001) and a higher probability of transitioning back to sedentary time ( <italic>P</italic>  = 0.027). Rest–activity rhythms were less stable, with lower interdaily stability ( <italic>P</italic>  = 0.030), greater intradaily variability ( <italic>P</italic>  = 0.008), lower M5 activity ( <italic>P</italic>  &lt; 0.001) and later M5 timing ( <italic>P</italic>  = 0.015) and cosinor acrotime ( <italic>P</italic>  &lt; 0.001). Most activity and rhythmicity outcomes were similar in the DS and obesity groups. Participants with DS slept longer but had poorer sleep continuity. Overall 24-hour behavioural profiles differed significantly among groups (P &lt; 0.001). Conclusions Children and adolescents with DS display a sedentary, low-movement and less stable 24-hour behavioural profile resembling that observed in obesity. Integrated assessment of activity, sedentary accumulation, sleep continuity and rhythmicity may support early identification of modifiable risk and guide individualized, family-centred interventions. </p>