MONDAY, April 11 (HealthDay News) -- Normal daily fluctuations in blood pressure aren't linked with the well-documented fact that people are more likely to suffer a heart attack or stroke in the morning, a new study shows.

It included 28 people without high blood pressure who underwent three types of sleep/wake cycle experiments designed to assess the connection between the internal body clock and the daily rise and fall in blood pressure.

In all three experiments, the volunteers showed an internal daily blood pressure variation with a peak at about 9 p.m. and the lowest blood pressure occurring in the late morning.

The researchers said the surprising finding that blood pressure is lowest in the morning means it's unlikely that the internal blood pressure cycle is connected to the increased risk of heart attack and stroke in the morning.

The study was published April 7 in the journal Circulation.

"We used three complementary experimental protocols and three different groups and found essentially the same results. That means we're dealing with something very robust," lead author Steven A. Shea, an associate professor of medicine at Harvard Medical School, said in a journal news release.

More information

The American Academy of Family Physicians has more about heart attack.