Restorative sleep is expected to promote improved endothelial function (EF) in the morning compared to the evening. However, in adults with obstructive sleep apnea (OSA) EF is not only adversely affected, but it worsens during the night. Data in pediatric OSA are scarce, and overnight changes have not been explored. Therefore, we sought to examine potential associations between pediatric OSA and overnight changes in EF. 59 habitually snoring children with various degrees of sleep-disordered breathing (age range, 4-16 years) underwent EF assessment (reactive hyperemia test by EndoPAT, Itamar Medical, Israel) in the evening before and the morning after an overnight polysomnography (PSG). Two brachial occlusion periods (1 min and 5 min) also were tested. Potential associations between evening-to-morning changes in EF and polysomnographic parameters were explored. Evening-to-morning changes in children with OSA displayed severity-dependent deterioration of EF, and occlusions lasting 1 or 5 min during the reactive hyperemia test yielded similar findings. In children deterioration in EF during the night significantly correlated with the severity of OSA. Furthermore, the reactive hyperemia test can be reliably performed with only 60 seconds of arterial flow occlusion in children. These findings support our hypothesis that similarly to adults, sleep apnea in children results in endothelial dysfunction (ED). We speculate that pediatric OSA is less commonly associated with cardiovascular complications possibly due to the shorter duration of the syndrome.