A new study demonstrates, for the first time, that a single session of moderate-intensity exercise can elevate your mood. Among the “moderate-intensity” options: something as simple as a 30-minute walk. The study, published in the December issue of Medicine & Science in Sport & Exercise, was done on people with major depressive disorder. But the researchers said they believe that the single-session boost in spirits also seems to apply to people who just feel a little down.