For people who have recently had a stent implanted in a blocked heart artery, the risk of developing a blood clot may be higher early in the morning than other times of day, researchers reported Monday.

Stents are tiny, scaffold-like devices implanted into narrowed heart arteries to help improve blood flow to the heart. In about 1 to 2 percent of patients, a blood clot forms around the stent, which can trigger a heart attack or stroke; the risk is greatest in the few months after the stent is implanted.

In the new study, researchers found that for people who developed a blood clot within a month of their stent procedure, symptoms were more likely to begin at 7 a.m. than at any other time of the day.

Among the 49 patients, symptoms arose at 7 a.m. at least twice as often as any other hour of the day, according to findings published in the Journal of the American College of Cardiology: Cardiovascular Interventions.

The study adds to evidence that the risk of heart problems varies according to the body's circadian rhythms, the 24-hour cycles that influence sleep, hormone production, body temperature and other essential functions.

Past studies have found that the incidence of heart attack, cardiac arrest and stroke generally peak in the morning.

"We expected to see circadian patterns and variations (in blood clot risk), because they are part of normal life," said Dr. David R. Holmes of the Mayo Clinic in Rochester, Minnesota, the senior researcher on the study.

It's known that blood pressure, heart rate and the blood cells' "stickiness" tend to increase in the morning, all of which might contribute to the higher risk of stent blood clots, according to Holmes's team.

A researcher not involved in the study called the findings "interesting" and said that future studies are needed to confirm the greater morning-time risk and uncover the reasons.

"Identifying this circadian pattern helps focus attention on the possible reasons," said Dr. Geoffrey Tofler, a professor of preventative cardiology at the University of Sydney in Australia who has studied circadian rhythms and heart disease.

But what can patients with newly implanted stents do for now?

Holmes said they can be particularly attuned to potential symptoms of a blood clot early in the morning. Those include chest pain or discomfort and shortness of breath. There are also "atypical" symptoms, like nausea, Holmes noted, and elderly patients are more likely to have those compared with younger people.

"If you wake up feeling 'strange,'" Holmes said, "maybe you're not just feeling strange, and it's something that needs medical attention."

Still, since stent blood clots arise in about 1 to 2 percent of patients, the absolute risk to any one person would be low. And Holmes stressed that, in general, "stents are safe and effective, and very good at preventing symptoms."

"But they are not fail-safe," he said. "And they need careful medical control."

Stent patients generally take aspirin plus another anti-clotting medication for some time after the procedure, a year or more if they have drug-coated stents. So staying with your prescribed regimen is important, both Holmes and Tofler pointed out.

The findings are based on 124 patients treated at the Mayo Clinic for a stent blood clot between 1995 and 2009. Of those patients, 49 had an "early" clot, within 30 days of having the stent implanted, while 30 developed a clot within a year, and 45 more than one year later.

Only the risk of an early clot was elevated in the morning; later ones showed no clear circadian pattern.

That, according to Holmes, may be because early and late blood clots happen for different reasons.

The researchers were also able to pinpoint potential triggers of blood clots in some of the patients. About 6 percent had not been complying with their medication, and another 8 percent had infections or medical problems that could have either contributed to the clot or interfered with their medication regimen.

In addition, 11 percent of the patients said they'd been performing heavy exercise before their symptoms arose something that is not advised.

"It is generally recommended that people not perform heavy exertion in the initial weeks after receiving a stent, since this may increase risk of stent thrombosis (blood clot)," Tofler told Reuters Health by email.

According to Holmes, the findings also raise the question of whether patients' anti-clotting medication doses may be wearing off by the early morning, and whether it might be better to split the dose into two, taken in the morning and night.

Tofler, however, expressed doubts about that. The "duration of action" of both aspirin and clopidogrel (Plavix) the anti-clotting drug commonly given to stent patients is longer than one day, he said.

"So unless people weren't taking those tablets when they should have been," Tofler said, "timing should not have been a problem."