Desperate for rest in a frenzied world, at least 8.6 million Americans take prescription sleeping pills to catch some Zzzs, according to the first federal health study to focus on actual use.Between 2005 and 2010, about 4 percent of U.S. adults aged 20 and older popped popular prescription drugs such as Lunesta and Ambien in the previous month, say government researchers who tracked 17,000 people to their homes and peered into their medicine cabinets.