According to Dr Ibhar Al Mheid, a postdoctoral cardiology researcher at Emory, endothelial cells serve as the interface between the blood and the arteries, controlling arterial tone and helping to prevent clots that lead to strokes and heart attacks.
The researchers sought to determine the circadian pattern of both endothelial function -- the ability of blood vessels to relax -- and the abundance of the progenitor cells.
"One of the important ways the lining of our blood vessels is maintained is by progenitor cells that come from the bone marrow," said Al Mheid.
"These are essentially stem cells that help replace endothelial cells at sites of injury and build new vessels at sites deprived of adequate blood supply," Al Mheid added.
During the study, the researchers examined the a dozen healthy middle-aged subjects every four hours for 24 hours.
Blood vessel relaxation was assessed by cuff occlusion, a standard technique in measuring blood pressure.
The researchers measured the ability of subjects' blood vessels to relax, the abundance of endothelial progenitor cells (EPCs) and their ability to grow in culture.
They found that both the ability of blood vessels to relax and EPCs' ability to grow peaked at midnight, while endothelial function was weak in early morning.
"The lining of our vessels appears to function better at night than in the day. Endothelial function is particularly depressed in the early morning hours," said Al Mheid.
The study was presented at the American Heart Association Scientific Sessions in New Orleans.