Not getting enough sleep at night could dramatically increase a person's risk of heart attack or stroke.  A new preliminary study shows people over the age of 45 who sleep less than six hours every night suffer double the risk of having a heart attack or stroke.  They're also at 70-percent higher risk for congestive heart failure. However, getting too much sleep can be dangerous as well.  Researchers say people who get more than eight hours a night are at higher risk for coronary artery disease and angina.  The study concludes people should aim for about seven to eight hours of sleep every night.  The study was presented at the American College of Cardiology's annual meeting in Chicago.