Imagine not drinking all day at work—no coffee, no water, no diet soda. At the end of an 8-hour shift, you'd be pretty parched. Which is precisely why you should start rehydrating immediately after a full night's slumber. From now on, drink at least 16 ounces of chilled H2O as soon as you rise in the morning. German scientists recently found that doing this boosts metabolism by 24 percent for 90 minutes afterward. (A smaller amount of water had no effect.) What's more, a previous study determined that muscle cells grow faster when they're well hydrated. A general rule of thumb: Guzzle at least a gallon of water over the course of a day.