I know the feelin'!

It's a form of mild shock, the body's natural reaction to threat. Your brain gets flooded in a cocktail of happy chemicals (adrenaline, cortisol, et cet) that makes you feel euphoric and painless.

Too much of it will **** you up, but in moderation I think it's actually good for you.

I hope so, anyway, cause that's a large part of why I like to ride my bicycle....