Background— Cardiovascular events such as myocardial infarction, sudden death, and stroke have a peak incidence in the early hours after waking. The mechanisms involved in this circadian variation are not clear. Endothelial dysfunction is associated with increased risk for cardiovascular events. We tested the hypothesis that endothelial function is reduced in the early morning, around the time of waking, compared with measurements obtained both before sleep and later in the day in healthy humans.

Methods and Results— We studied 30 subjects (19 men, 11 women; mean age, 41.6 years). All participants underwent polysomnography to exclude obstructive sleep apnea or other sleep disorders. Brachial artery flow–mediated endothelium-dependent vasodilation (FMD) and endothelium–independent dilation (non-FMD) were measured on 3 different occasions: before subjects went to sleep (9 pm), the next morning immediately after waking (6 am), and during the late morning 5 hours after waking (11 am). All subjects had normal sleep with good sleep efficiency of 84±2%. Compared with before sleep, FMD decreased markedly in the early morning after waking and recovered by late morning (9 pm, 7.5±1%; 6 am, 4.4±0.7%; 11 am, 7.7±1%; P=0.02). Non-FMD was similar for the 3 periods of observation (9 pm, 17.3±1.6%; 6 am, 17.2±1.3%; 11 am, 18.5±1.7%).

Conclusions— FMD is blunted in the early morning in healthy subjects. Decreased endothelial function in the early morning may have implications for our understanding of the morning peak in cardiac and vascular events.