WEDNESDAY, May 14 (HealthDay News) -- A daily aspirin can control prehypertension, but only if it is taken at bedtime, a Spanish study shows.

An aspirin taken every morning didn't lower the blood pressure of prehypertensive people, but the evening regimen did, Dr. Ramon C. Hermida reported Wednesday at the American Society of Hypertension annual meeting, in New Orleans.

A previous study by Hermida, who is director of bioengineering and chronobiology at the University of Vigo, showed the same beneficial effect of bedtime aspirin for people with moderately high blood pressure. The new report is the first study to show the drug's benefit -- although only when taken at night -- with prehypertension, defined as blood pressure just below the 140/90 level. Prehypertension is a known warning sign of future risk of heart disease, stroke and other cardiovascular problems.

Why aspirin should do its good work for blood pressure at night but not in the daytime is not clear, Hermida said. Research indicates that it can slow the production of hormones and other substances in the body that cause clotting, many of which are produced while the body is at rest.

The three-month study included 244 adults diagnosed with prehypertension. A third of them were advised to follow general rules of hygiene and diet designed to reduce blood pressure, another third were told to take a 100-milligram aspirin tablet every night at bedtime, and the final third were told to take the same aspirin dose on awakening.

Researchers monitored blood pressure levels at 20-minute intervals from 7 a.m. to 11 p.m. and at 30-minute intervals at night before the trial began and three months later.

Systolic blood pressure (the higher number in the 140/90 reading) dropped 5.4 points and diastolic pressure by 3.4 points for those taking aspirin before bedtime. No drop in blood pressure was found in those taking morning aspirin or following the general guidelines.

"There is some evidence that taking a variety of medications, including those for hypertension, at night is associated with greater blood pressure reduction than taking them in the morning," said Dr. Suzanne Oparil, president of the American Society for Hypertension. "I don't think we know why."

It's possible that there might be better absorption of the medication by the gastrointestinal tract at night, said Oparil, who is a professor of medicine at the University of Alabama at Birmingham.

"It's all a little bit speculative about why, but I think the observation is solid," she said.

In a statement, Hermida said the new findings "show us that we cannot underestimate the impact of the body's circadian rhythms."

"The beneficial effects of time-dependent administration of aspirin have until now been largely unknown in people with prehypertension," he said. "Personalizing treatment according to one's own rhythms gives us a new option to optimize blood pressure control and reduce the risk of cardiovascular disease down the line."