What is runner's high?
Plenty of people enjoy running, but not everyone experiences the state of euphoria known as runner's high that sometimes comes when you are doing vigorous exercise.
It's a hard concept to define. Some people describe it as a feeling of total wellbeing, you don't feel tired and become unaware of time passing.
"There's a lot of debate about this euphoric state which long distance runners, swimmers and cyclists experience," says Amanda Wilding, senior lecturer in Sports Psychology at the University of Bournemouth.
She says: "It's something to do with the rhythmic manner and the distance travelled which induces this high."
What causes it?
It's not known exactly what causes runner's high. Endorphins are thought to play a role. They are the body's natural feel-good chemicals and doing robust exercise produces more of them.
A 2008 German study of long distance runners found that vigorous exercise produced endorphins and the runners who felt the most euphoric after a run had higher endorphin levels in their brains.
Endorphins may act as a natural painkiller and reduce the discomfort and tiredness of achy legs and sore feet.
It may be that exercise produces other substances in our bodies, like dopamine and serotonin, which make us relax and have been shown to boost our mood.
"It seems to be a combination of physiological, biological and psychological variables that come together at the same time that gets you in the zone but a little more so," says Amanda.
When does it happen?
People experience runner's high differently and at different stages of their run, some at 5 miles others not until they have clocked up 20 miles.
"Runner's high is different from the buzz you sometimes get during or just after exercise," says marathon runner Chris Morgan, 47, from Cardiff.
"I've experienced it occasionally and only on long training runs of over 16 miles or during actual marathons. The key thing I find is that it usually follows a spell where I have struggled for a bit and my pace had dropped. Suddenly out of the blue I feel ecstatic and energised and usually start running a bit quicker."
When you are in the zone and in this state of euphoria you may not even be totally aware of it.
"People tend to realise they've experienced it afterward," says Amanda. "They talk of floating and being in an altered state or very motivated and energised and in a slightly different time continuum."
Are there degrees of runner's high?
Not every runner or individual who does other endurance exercise experiences this total feeling of elation. It may be down to your individual make up, you may not be running far enough or working hard enough. It may be the case that all of the factors that contribute to runner's high may not yet have been aligned in your case.