Research shows that exercising outdoors lowers stress levels far more effectively than at a gym. And if you've ever felt exhilarated after a snow-whipping run or a cold ski, you know this kind of exercise can be addictive – another reason why people who work out outside are more likely to stick with it than gym-goers. "Being outdoors motivates us to do activities regularly," says lead researcher Michael Depledge. Outdoor exercise can burn up to 20 percent more calories than indoor exercise because your body has to fight to stay stable on uneven ground and maintain its temperature in all kinds of weather. Most people also get a harder workout in nature, since their focus is on the effort, not a TV, an iPhone, or another distraction used to ease gym boredom. One Harvard study even found that outdoor exercise cuts heart disease by boosting vitamin D levels.