My wife once told me that she notices I'm a less happy person if I'm not regularly playing sport. Since she mentioned that, I've been able to sensitise myself to when I think I'm starting to feel irritable for reasons of 'not having competed recently'. I've found it to be quite tangible.

There's definitely an evolutionary element to this, something along the lines of a requirement to contribute to the survival of the tribe; prove ones worthiness.

The other thing about sport, for me, is singular focus. Whilst it's physically draining, it's a temporary escape from all those other niggling thoughts and helps to get rid of the trivial ones. Meditation-esque.