Most of us can think of dozens of reasons not to exercise. You're already tired; how can you summon the energy to move more? It takes time out of a busy day. It's cold out. Your knee hurts.

But the evidence keeps rolling into my inbox. Researchers have found a few more reasons why we should do it.

Some of the most impressive evidence concerns the ability of exercise to keep our moods stable. Study after study shows that exercise combats depression. It lifts your mood, restores your energy, realigns your brain chemistry—and the price is unbeatable. It costs nothing. Physical activity works at least as well against mild to moderate depression as any other treatment.

Exercise also changes your perception of yourself. It provides a sense of personal mastery and positive self-regard.

We're not talking here about Olympian levels of activity. When it comes alleviating depression, it's not at all necessary to go for the burn. All it takes is 30 minutes of aerobic exercise three times a week. That means walking. Researchers at Duke University have found that 50 minutes of exercise a week brings about a 50% decrease in the likelihood of being depressed.

Now more news of what exercise can do. It boosts blood flow to the brain, which allows you to be more mentally engaged. Exercise not only gives you physical energy, it boosts your mental energy. It makes you more alert. These benefits, need I point out, are aside from the ability of exercise to protect the heart and balance body weight.

Studies were performed on a few dozen monkeys that were put on a treadmill for five days a week over 30 weeks. They were compared with animals who remained sedentary.

On tests of mental performance—the animals had to find a treat placed under toys—the exercisers shone. "Tests showed that animals in the exercise group were more aroused, alert and engaged than animals in the control group," the researchers reported recently. What's more, the exercisers learned how to navigate the tests of mental performance at a much faster rate.

So do yourself and your brain a favor, go out for a walk.

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