Background <p>Recently, poor sleep was closely related to physical function decline in older adults. To investigate the possible mediation pathway between them, the relationships among sleep status, lower limb composition, and physical performance were analyzed.</p> Methods <p>The cross-sectional study included 323 adults (46% women) aged 60 years and above (mean age 71.7 years) from the community of China. Sleep duration and sleep satisfaction were collected by questionnaire. The composition of lower limbs was quantified by dual-energy X-ray absorptiometry. Gait speed was measured as a physical performance indicator. The mediation role of low lean mass and high fat mass in the relationship between sleep status and physical performance was analyzed by bootstrap mediation analysis.</p> Results <p>The average sleep duration of participants was 6.8&#xa0;h, about 43% participants were dissatisfied with their sleep. Compared with medium sleep duration, long sleep duration was associated with low lean mass (OR 2.630, 95% CI 1.390–5.013), high fat mass (OR 2.151, 95% CI 1.155–4.010), and slow gait speed (OR 3.143, 95% CI 1.700–5.880). Participants with long sleep duration and unsatisfactory sleep had the worst lower limb composition and physical performance among them. The mediation effect of lower limb composition on the relationship between long sleep and gait speed was significant, with 15.3% of the total effect being mediated by lean mass and 19.9% mediated by fat mass.</p> Conclusions <p>Long and unsatisfactory sleep had a great relationship with poor physical performance, and the change of lower limb composition was one of the mediate factors involved in this relationship. The combination of sleep duration and sleep satisfaction together was a good indicator for sleep assessment, which provide a new strategy for the intervention of physical function decline in older adults.</p> Clinical trial number <p>Not applicable.</p>