Don’t feel guilty about sneaking in a nap now and then: It might be good for your heart. People who napped once or twice a week were 48 percent less likely than non-nappers to face serious cardiovascular problems — heart attack, stroke, heart failure — according to new research. The findings, published in the journal Heart, were based on nearly 3,500 adults, ages 35 to 75, who were tracked for about five years. How long people napped each time made no difference, and napping more frequently than a couple times a week did not improve the results. Sleep experts generally agree that a ­20-minute nap is all that most people need to feel refreshed and less stressed. Napping longer means waking from a deeper sleep and that can leave someone feeling groggy or fuzzy-headed. Napping late in the day also is not recommended because it can mar nighttime sleeping. The recommended amount of sleep for most adults is at least seven hours a night, with an hour or two more for people 61 and older. According to the Centers for Disease Control and Prevention, more than a third of Americans regularly get too little sleep. That can lead to chronic health problems, including diabetes, high blood pressure and heart disease, according to experts at Harvard Medical School’s Division of Sleep Medicine.