Just 10 minutes of moderate exercise may improve your mood, a study suggests. Once a week for four weeks, 14 female students at Northern Arizona University joined one of four activities: pedaling an exercise bicycle at a moderate pace for 10, 20 or 30 minutes, or sitting quietly for 30 minutes. Before and after each test, the women filled out a mood questionnaire. Ten minutes of moderate exercise improved mood, increased vigor and decreased fatigue, according to a study published in 2003 in Health Psychology. Working out longer didn't offer a greater change in mood (although it may offer more health benefits).