How do you feel after a good workout? Tired, a bit muscle sore perhaps? Most people will probably also add "happy".

Your improved mood is not an individual phenomenon. Plenty of studies have found that exercise can trigger happiness, tranquillity, euphoria and creativity for anything from a few minutes to 24 hours.

Those who exercise regularly often describe themselves as "addicted" and describe restlessness and depressed mood if they cannot indulge in their favourite exercise for a few days.

What is it that causes the mood enhancing effect of exercise?

Endorphins

Popular belief has it that when we exercise the brain produces an excess of what are known as endorphins. These are substances which are chemically related to morphine and other opiates and act by blocking pain receptors in the brain.

The idea was that exercise produced a "natural high", similar to that from opiates, and it was this effect which resulted in the good mood.

Certainly vigorous exercise, even for short periods, causes blood endorphin levels to rise far above their normal levels for several hours, although no one knows why this happens.

But is this what causes the exercise high?

Some studies have shown that drugs which block the effects of opiates, and so endorphins, reduce this effect when given to volunteers. But an equal number of studies have shown no effect at all.

Attempts to relate mood to blood levels of endorphins have also not been successful. In one study a psychologist measured mood and endorphin levels in 28 women on an eight-week exercise programme. Endorphin levels certainly increased in some women, but not necessarily the most in those who experienced the greatest exercise highs.

Why does exercise improve mood?

Two theories are the "mastery" hypothesis and the "distraction" hypothesis. In the "mastery" hypothesis the idea is that exercise enhances mood by giving you a sense of achievement. The "distraction" hypothesis suggests that exercise takes you mind off daily stresses.

However, plenty of other things can leave you with a sense of achievement and distract you from stress without giving you the same lift that exercise can.

Some people believe that the increase in body temperature which occurs with exercise improves mood. This effect has been noticed in people taking saunas, without any exercise.

However, a study in which volunteers ran on a treadmill in different temperatures showed that those whose body temperature increased experienced an increase in anxiety. When their body temperature fell to normal their anxiety levels also decreased. However, brain temperature may be what is important, and this is regulated separately from the temperature of the rest of the body.

Others think that it is not the exercise itself which makes you feel good, but where you go to exercise. Volunteers who ran on a treadmill listening to a tape of outdoor sounds, merely felt tired at the end of an exercise session. This was in contrast to those who ran on an outdoor track and did experience an improvement in mood.

So maybe it is not simply exercise, but the kind of exercise and where you do it that is important.

The energising effect

Those who exercise also feel energised. Relaxation and deep breathing can release tension, but rarely result in increased energy levels.

What you think about as you exercise can also have an effect. In a study of 150 runners before and after a training run, those who thought about their external surroundings during the run felt invigorated afterwards, while those who thought about their relationships felt less tense and anxious.

Part of the psychological high after exercise can be attributed to achieving goals. The greater the challenge of the exercise, the greater the sense of achievement.

What does all this mean?

Although the idea that endorphins are responsible for the ability of exercise to enhance mood is shaky, it is likely that a combination of psychological and physical factors will be found to account for this phenomenon.

In the meantime, there is good evidence to suggest that exercise does enhance mood, and some psychologists and psychiatrists are even prescribing exercise as part of the treatment for depression and anxiety, with excellent results.

One Norwegian psychiatrist prescribes as little as three one-hour sessions per week of either brisk walking or jogging for eight weeks. The patients apparently prefer this to drugs or psychotherapy.

Depression and anxiety are very common, affecting one in four people in the developed world. Figures are likely to be similar in the developing world. Many people are now promoting exercise as a means of reducing the incidence of these illnesses.

You don't have to be clinically depressed to benefit from exercise. A regular exercise program will increase your confidence, energy levels and general feelings of well-being. Try it!