One of the best ways to relieve the stresses of the day is to simply walk them away, scientists have found.

Walking briskly or jogging really does calm you down by sparking nerve cells in the brain that relax the senses, new research has shown.

Exercise has long been thought to be effective for anxiety and depression, but the brain mechanism¹ behind the phenomenon has remained a mystery.

Now tests on mice show when they are active, soothing² neurons in the brain are triggered. When they are then exposed to a stressor, these neurons calm them down.

Researchers took two groups of animals, one that ran around and another that was sedentary, and then tested the amount of brain cells that formed after they exercised.

The runners' brains had a number of new neurons specifically designed to inhibit³ brain activity and quiet more excitable ones. The researchers then put the mice into ice cold baths to create a stressful environment.

As expected, large numbers of excitable neurons were fired up in the hippocampus, a region of the brain involved in emotional responses.

But in the brains of the runners, calming neurons were also quickly released warding⁴ off unnecessary anxiety.

The research shows exercising mice are able to cope better with stress, and it is believed the effect is also seen in humans, reports the Journal of Neuroscience.

Professor Elizabeth Gould, of Princeton University, New York, said physical activity reorganizes the brain so anxiety is less likely to interfere⁵ with normal function.

She said: 'Understanding how the brain regulates anxious behavior gives us potential clues about helping⁶ people with anxiety disorders⁷.

'It also tells us something about how the brain modifies itself to respond optimally⁸ to its own environment.'

The findings resolve a paradox⁹ because exercise reduces anxiety while also promoting the growth of new neurons in the hippocampus.

Because these young cells are typically more excitable than their more mature counterparts, exercise should result in more anxiety, not less.

But the researchers found exercise also strengthens the mechanisms¹⁰ that prevent these brain cells from firing.

Prof Gould said identifying neurons and regions important to anxiety regulation may help scientists better understand and treat human anxiety disorders.

From an evolutionary¹¹ standpoint, the research also shows the brain can be extremely adaptive and tailor its own processes to an organism's lifestyle or surroundings.

A higher likelihood of anxious behavior may have an adaptive advantage for less physically¹² fit creatures.

Anxiety often manifests itself in avoidance behavior and avoiding potentially dangerous situations would increase the likelihood of survival, particularly for those less capable of responding with a 'fight or flight' reaction, added Prof Gould.