It's been well known for sometime that exercise can be very addictive.  The release of endorphins and all that comes with hard exercise create a feeling of euphoria and happiness.  So far so good.  What I didn't know however was that there may well be evolutionary reasons why exercise is addictive.

It all centres around a brain chemical called endocannabinoids.  When released in the brain they make us feel happy.  You can probably guess from the name that the chemical works in a similar way to how cannabis works on the brain (but in a much healthier way).

Now research suggests that evolution has ensured that this chemical is released in the brain when we run because in an evolutionary sense, running has been pretty important to the survival of mankind.  Hunting for food or running away from danger, that sort of thing.

Apparently it also exists in other animals as well, but only in those for which there is an evolutionary advantage in running well.  Sloths for instance don't tend to have endocannabinoids, but horses do.  What's interesting is that the chemical is only released during running.  Walking doesn't do it.  Cycling doesn't do it and neither does swimming.

Given that mankind generally doesn't need to run any more in order to catch food or avoid predators, it would seem likely that our sedentry lifestyle could see us lose the so called runners high.