So in many worlds, magic is a source of power that magic users can draw upon and "do stuff with" - make elemental effects (fire, cold, lightning), heal wounds, shield from arrows, light rooms, etc. The amount of power they can draw upon and the skill with which they can turn it into useful stuff is like any other talent: they have some innate ability that is improved by practice/training/studying. 

Given this form of magic, what if magic users couldn't turn that source off? The energy would need to go somewhere. How would magic users deal with this, even as babes or when sleeping?