Doing mental or physical work while fatigued may lead to hypertension and heart disease, suggests a U.S. study.

It included 80 volunteers who were told they could win a prize by memorizing, in two minutes, a number of meaningless three-letter sequences. Their blood pressure and heart rate were monitored while they tried to memorize the information. Those with moderate fatigue showed stronger blood pressure increases than those with low fatigue, United Press International reported.

The study appears in the July issue of the International Journal of Psychophysiology.

The University of Alabama at Birmingham researchers said their findings support a theory that a fatigued person’s cardiovascular system has to work harder when trying to complete tasks, UPI reported.

“Individuals who experience chronically exaggerated cardiovascular responses are believed to be at greater health risk than individuals who do not. Thus, the implication is that chronic fatigue may pose a health risk under some performance conditions,” said study leader Rex Wright.