While the benefits of exercise to the heart are known to increase during the first hour, continued exertion after the first hour could be detrimental, possibly causing heart rhythm problems or even scarring. Research published in the Mayo Clinic Proceedings suggests that marathon runners, triathletes and long-distance cyclists may be damaging their hearts through excessive endurance exercise. The doctors involved approve of exercise in general, but advise that challenges like marathons be short-term goals, rather than long-term exercise patterns. Regular exercise has been linked to lower rates of disability and longer life expectancy, but no additional benefits are gained beyond 30 to 60 minutes a day of vigorous activity. Ideally, running should be done two to three times a week maximum, alternated with other activities like yoga, walking and swimming. Those who are overweight are advised to spend 60 to 90 minutes a day involved in less intense exercise like walking or light bicycling.