You can certainly get by drinking less water—75 percent of Americans are “chronically dehydrated” and most average about four glasses a day, according to recent research—but more is definitely better. Eight glasses is a great goal, because water promotes satiety, prevents heart attacks, builds muscle, and stokes your metabolism. That last one is especially true in the morning, when your body has just spent the equivalent of a day in the office without any intake of fluids whatsoever. In fact, German scientists discovered that drinking at least 16 ounces of chilled H2O as soon as you roll out of bed boosts metabolism by 24 percent for 90 minutes afterward, and researchers at the University of Utah in Salt Lake City have also found that volunteers who drank at least eight glasses daily had higher metabolic rates than those who sipped only four glasses. So if your glass is half full—fill it up!