Daniel I. Galper, senior research associate at the University of Texas Southwestern Medical Center in Dallas, has performed a meta-analysis of 1,000 studies on the effects of exercise on mood. His observations about practical applications of these effects:

* Thirty minutes of moderate-intensity exercise at least five days a week -- which happens to be the Centers for Disease Control and Prevention's recommendation for all adults, regardless of their mood or psychological state -- "should be enough to have an effect" for most people with depression, Galper said. Some studies have shown that 20 to 30 minutes per day three days a week resulted in lowered depression, "but that hasn't been solidly demonstrated," he said.

* In a reversal of the usual outcome, where those who are fitter benefit more than others from an increase in exercise, "very unfit people will get more of a response from less exercise" than will more-fit people, Galper said. Why? Someone who's out of shape has to work harder just to walk, for example, for 15 minutes; that extra effort will activate more physical and cognitive hardware.

* A single session of exercise can boost energy level and mood for two to four hours. Broader, lasting effects on mood may require 10 weeks of workouts. "Sixteen weeks or longer showed the best effect," Galper said, "but even irregular or intermittent exercisers will see some mental health benefit."

* Other things being equal, and assuming a person can develop sufficient strength and cardiovascular capacity, intense workouts provide more benefits than less-strenuous ones.

-- John Briley