N owadays, most people realize that the mind and body are intimately related. But we tend to believe that our state of mind determines how energetic we are. Actually, it’s the other way around. Energy drives mood.

There’s a close connection between fatigue and negative moods. Years of research have proven that when people lack energy, they become pessimistic, unhappy, prone to worry -- and even depressed. Small things bother them. Minor problems seem insurmountable and insoluble. Self-esteem sags.

On the other hand, high energy brings upbeat, positive moods. You smile, enjoy life, feel hopeful and happy. You think good thoughts about yourself and about others... and about the world in general.

The other half of the equation is tension. Up to a point, tension is energizing -- modest anxiety revs you up. But as stress rises, vitality ebbs. You get tired and anxious. Your mood turns sour. A vicious cycle begins.

MOOD MODULATION

Since moods help determine our enjoyment of life, we’re constantly trying to regulate them. Usually without knowing it, we do what we can to make ourselves feel better by raising energy and dispelling tension.

Often, we enlist positive activities, such as listening to music, calling a friend or reading a book. But it can also mean drinking or other ill-advised attempts at “self-medication.”

TENSE TIREDNESS

One common strategy people use to improve their moods is eating. It makes sense -- food is the source of energy, and eating soothes tension, too. Years of conditioning have trained us to reach for a snack automatically when we’re even mildly tense and tired, and we’re drawn to foods that are rich in calories, sugar and fat.

If this sounds like an unpromising equation, it is. Studies have shown that “tense tiredness” is the condition under which overeating tends to occur.

It is no coincidence that in recent decades -- as anxiety and stress have increased and the overall mood of Americans has worsened -- obesity has reached epidemic proportions.

Exercise offers a far more constructive way for you to raise energy, reduce tension and improve mood.

Even a brief period of modest activity -- walking briskly for five or 10 minutes -- has been shown time and again to boost energy substantially.

In one recent study, we found that after 10 minutes of brisk walking, energy was still significantly higher one hour later. Even two hours later, people who participated in the experiment felt somewhat more energetic than they had before.

The effect of exercise on anxiety and tension is less clear-cut, but it can be significant -- especially after a more intense workout, such as a session on an exercise bicycle.

Perhaps most important, exercise makes people feel better. After a bit of activity, most people report increased happiness. Many also say that exercise relaxes them.

Exercise helps even very bad moods. A recent Duke University study found that a regular program of physical activity relieved depression just as effectively as a widely used prescription antidepressant did.

TAKING CONTROL OF YOUR ENERGY

Why is it so hard to stick with a healthful exercise schedule -- and so easy to overeat?

When your mood and energy are low, exercise is the last thing you feel capable of doing. The idea of getting up and being active seems overwhelming. How much more appealing it is to reach for a snack... and since this does give you a temporary boost, it strengthens the connection. Which means you’re even more likely to do it next time.

To change the pattern, you must become more aware of your mood cycles.

What to do: For several days, start every hour by self-rating your energy and tension levels. Use a scale from one to seven. In a notebook, chart your own cycle. If you’re like most people, you’re likely to find your energy is low when you get up, rises through early afternoon, then falls drastically. There may be another slight rise in early evening.

Note your urges to eat -- and see how often the rich snacks you regret later are connected to low energy and high tension.

Now experience the effect of action. When your energy is low, go out for a brisk walk -- 10 minutes of striding quickly -- as if you’re late for an appointment, but without the anxiety. Do this no matter how much you don’t feel like doing it.

Carefully note what happens. You’ll probably feel a slight surge in vigor early on, and by the time you get back, you’ll rate your energy level considerably higher than when you left.

For most people, a brief, brisk walk can mean a quantum leap in energy, a jump up from its late afternoon low level to the equivalent of its midmorning peak.

GOING FORWARD

Put this knowledge to work for you. Each time you feel in your own body what exercise can do, you’ll find it easier -- next time you’re sluggish -- to dismiss negative thoughts, override inertia and get moving, not snacking. And that’s the road to a more lively life.