Many people say they don't work out because they simply don't have the time. If you're one of them, your favorite excuse may have just fallen by the wayside -- a new study shows just 30 minutes of daily activity is enough for weight loss.

The trick is to exercise hard enough to break a sweat. Do that, and you'll get better results in 30 minutes than many people get in an hour of more moderate exercise.

Researcher Mads Rosenkilde, a PhD student at the University of Copenhagen, says the study results were surprising, but could have something to do with shorter workouts leaving participants energized rather than exhausted -- which makes them more willing to exercise again the next day.

In addition, people who who did 60-minute workouts probably ate more to compensate for the extra time they spent at the gym, thus negating a lot of the work they did and slowing their weight loss.

“Another interesting scenario is to study exercise as a form of transport," said Rosenkilde. "Training is fantastic for your physical and mental health. The problem is that it takes time. If we can get people to exercise along the way – to work, for example – we will have won half the battle.”

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