A daily dose of laughter may be good for the heart because, like exercise, it makes blood vessels work more efficiently, U.S. researchers reported on Monday.

Depression, on the other hand, can raise the risk of dying from heart failure, a separate study found.

The two studies, presented at a meeting of the American College of Cardiology in Orlando, Florida, show how psychological factors can affect a person's health.

"We don't recommend that you laugh and not exercise, but we do recommend that you try to laugh on a regular basis. Thirty minutes of exercise three times a week, and 15 minutes of laughter on a daily basis is probably good for the vascular system," said Dr. Michael Miller of the University of Maryland School of Medicine in Baltimore.

Miller and colleagues at the school showed two movies, one humorous, one stressful, to 20 healthy volunteers and tested the function of their blood vessels.

The researchers specifically looked at the endothelium, the lining of the vessels, and found that blood flow was reduced in 14 of the 20 volunteers after stressful movie clips. But blood flowed more freely in 19 of the 20 when they laughed at funny movie segments.

Average blood flow increased 22 percent during laughter, and decreased 35 percent during mental stress, they told the meeting.

"The endothelium is the first line in the development of atherosclerosis or hardening of the arteries, so, given the results of our study, it is conceivable that laughing may be important to maintain a healthy endothelium, and reduce the risk of cardiovascular disease," Miller said in a statement.

Laughter may be almost as helpful as exercise, Miller said.

"The magnitude of change we saw in the endothelium is similar to the benefit we might see with aerobic activity, but without the aches, pains and muscle tension associated with exercise," he said.

In a second study, Dr Wei Jiang and colleagues at Duke University in North Carolina followed 1,005 heart failure patients and also tested them for depression.

Those with mild depression had a 44 percent greater risk of dying, Jiang told the meeting.

"This adverse association of depression and increased long-term mortality was independent of other factors, including age, marriage, cardiac function and the root cause of the heart failure," Jiang said in a statement.

"Approximately half of all patients with heart failure will die within five years of diagnosis, and we believe that our study appears to identify a group of these patients who are at a higher risk for dying."

Jiang said it is not clear why, but he said patients with depression tend not to exercise or take medications properly.

"Also, depressed patients tend to make unhealthy lifestyle choices in such areas as diet and smoking," she said.