Poor sleep quality significantly increases left ventricular mass in South Asian and African/African-Caribbean adults but shows no association in Europeans, according to analysis of 1,284 participants from the tri-ethnic SABRE cohort. African-Caribbean participants with poor sleep showed the strongest effect, with 9.1g/m1.7 higher left ventricular mass per unit decrease in sleep quality, followed by South Asians at 5.8g/m1.7. This finding reveals important ethnic disparities in how sleep affects cardiac structure, potentially explaining differences in cardiovascular disease burden across populations. Left ventricular hypertrophy represents early cardiac remodeling that predicts heart failure, arrhythmias, and cardiovascular death. The mechanism may involve differential inflammatory responses, autonomic dysfunction, or genetic variants affecting sleep-cardiovascular pathways across ethnic groups. These results suggest sleep interventions could offer targeted cardiovascular protection for at-risk populations, though causality remains unclear from this observational design. As this is a preprint awaiting peer review, the findings require validation through independent replication and mechanistic studies before informing clinical practice. The work represents an important step toward personalized cardiovascular prevention strategies based on ethnic background.