Personally, i think it's one of the most confusing things about korean. I have asked and researched this so much and it didn't make me that much smarter. From the info i have gathered -은/는 put emphasis on the action where -이/가 put emphasis on the subject.
I'm going to ask this here instead of starting a new question, so please tell me if i'm right or wrong:
난 잘래 - Is this like... "i want to sleeeep"?
내가 잘래 - And this like.. "it is I who wants to sleep, me me me, not you but ME"?
A little bit exaggerated maybe, buy just so it's clear :D