Heart attacks

A daily reduction in flexibility of blood vessels may help explain why strokes occur mostly in morning.

Mayo Clinic researchers have discovered a daily reduction in the flexibility of blood vessels, which may help explain why heart attacks and strokes occur most frequently in the early morning hours.

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The findings are published in Circulation: Journal of the American Heart Association.

"The human body maintains a balance of blood pressure and blood flow by expanding or contracting blood vessels. Much of this expansion is directed by a layer of cells called the endothelium, which lines the blood vessels. Risks of sudden death, heart attack and stroke are 30 to 50 percent higher in the early morning hours, so we wanted to determine whether the endothelium's effects are diminished in the morning," said Virend Somers, a Mayo Clinic cardiologist and lead author of the study.

The study examined the blood vessel expansion caused by the endothelium in 30 normal, healthy volunteers who were nonsmokers. Measurements were taken at bedtime, and then at 6 a.m. and 11 a.m.

"Doctors have known for years that the morning hours are prime time for cardiovascular events such as heart attacks, sudden death and stroke, but the reasons are unclear. These latest findings may provide clues to the clustering. In fact, the early morning function of the blood vessels in healthy young subjects approaches the levels seen in smokers and diabetic people," said Somers.

"We are not sure yet how these changes relate to people with cardiovascular disease, but this reduced morning function of the endothelium may be among triggers that could cause an event. We will continue to explore the underlying mechanisms," he added.