People with high blood pressure who get less than the standard amount of sleep may face an increased risk of heart disease and stroke, researchers reported Monday.

In a study of more than 1,200 Japanese adults with high blood pressure, the investigators found that those who slept for less than 7.5 hours each night were more likely to suffer a heart attack or stroke or die of cardiac arrest over a 4-year period.

At particular risk were “short sleepers” who failed to have a blood-pressure dip that normally occurs overnight.

The findings suggest that doctors caring for patients with high blood pressure should ask them about their sleep habits, the researchers report in the Archives of Internal Medicine.

A relatively small number of people are both short sleepers and have no overnight decline in blood pressure, note the researchers, led by Dr. Kazuo Eguchi of Jichi Medical University in Japan. Therefore, “it is a group that could be easily identified and monitored more closely.”

The findings are based on data from 1,255 men and women with high blood pressure who were between 33 and 97 years old; the average age was 70. The researchers used portable blood pressure monitors to record each participant’s blood-pressure changes over a 24-hour period, and then followed the patients over an average of 50 months.

During that time, there were 99 heart attacks, strokes or deaths from cardiac arrest.

In general, Eguchi’s team found, men and women who slept for less than 7.5 hours per night had a 68-percent higher risk of one of these cardiovascular complications than their better-rested counterparts.

Similarly, the risk was greater among participants who had shown no drop in their overnight blood pressure, versus those who had.

But the combination of short sleep and non-dipping blood pressure conferred the highest risk—a more than four-fold greater chance of heart attack, stroke or cardiac death.

Chronic sleep deprivation has been linked to a higher risk of a number of health problems, including diabetes, coronary heart disease and obesity.

The researchers suspect that a lack of sleep leads to increased nervous system activity throughout the day, which may in turn stress the cardiovascular system. Non-dipping overnight blood pressure has also been tied to increased nervous system activity during the day, Eguchi’s team notes.

Therefore, they suggest, the combination might have an “interactive effect to increase cardiovascular risk.”