Poor sleep efficiency is associated with higher cardiovascular risk. Vascular endothelial function (VEF) is a non-invasive, nitric-oxide-mediated biomarker of cardiovascular disease assessed in the brachial artery by flow-mediated dilation (FMD). Prior work found an association between sleep efficiency and FMD in people with hypertension. The relationship between sleep efficiency and FMD in people without chronic diseases is unclear.