Exercise can indeed boost people's moods, but they may have to start off down in the dumps to feel the effects.

Researchers found that an hour of aerobics reduced tension, anger and fatigue among their study participants, with the benefit being significantly greater among those who felt depressed before the exercise bout.

A number of studies have shown that exercise can lift a person's spirits, and there is even evidence that physical activity can aid in treating clinical depression. The current study focused on "depressed mood," rather than clinical depression.