The hot new regimen for heart health isn�t kettlebells or high-intensity intervals. It�s strengthening what some researchers call your �optimism muscle.� People who look on the bright side are 50 percent less likely to have a heart attack or stroke, finds a new research review in the journal Psychological Bulletin. And while scientists are still nailing down exactly how mental well-being leads to physical health, they do believe a sunny disposition is something that can be built with effort.