i think it’s a dopamine thing. some doctors believe that our dopamine receptors aren’t that awesome. 
– thats a technical term, btw- i’m so scientifically literate, hehehe.
dopamine is a brain chemical that is implicated in motivation, pleasure, reward, impulse control, memory, attention, problem solving, decision-making, sociability, addiction- all sortsa stuff…. and also fruit going brown, according to wikipedia. i don’t think thats entirely relevant here though. i have no special powers over fruit spoilage as far as i’m aware. 
a lot of the ADHD medications give your dopamine levels a bit of a kick up the bum, so to speak- and having more of that stuff floating around in there doing it’s dopamine-y-thing, makes us a bit less ditzy and a bit more functional.
…. getting a load of decent exercise increases dopamine levels too- which is probably why it makes a difference for MarkJ when it comes to mood-related stuff.