WASHINGTON, March 7 (Reuters) - 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...