Sleeping between eight and nine hours per day reduces the risk of cardiovascular disease in humans.

Study: Association of Objective and Self-Reported Sleep Duration With All-Cause and Cardiovascular Disease Mortality: A Community-Based Study.

This study found that sleeping 7 to 8 hours a night is best for heart health, but sleeping more than 8 hours might actually be bad for your heart — so the claim that 8–9 hours helps isn’t fully right, since 9 might be too much.

Study: Associations of Accelerometer-measured Sleep Duration with Incident Cardiovascular Disease and Cardiovascular Mortality.

This study found that people who sleep 7 to 9 hours a night have lower heart disease risk than those who sleep less, and sleeping more than 9 hours isn’t worse — so sleeping 8 to 9 hours is likely safe and helpful for your heart.

Study: Causal association between sleep duration, daytime napping, sleep disorders and ischemic heart disease: A systematic review and meta‑analysis of Mendelian randomization studies

This study found that people who sleep longer — especially more than 6 hours — have a lower risk of heart disease. So sleeping 8–9 hours, as the claim says, likely helps protect the heart.