I don't know it's origination, but I suspect the 6+6 was probably a result of having the ability to split operations of a 24 hour day into even crews (not much

I don't know it's origination, but I suspect the 6+6 was probably a result of having the ability to split operations of a 24 hour day into even crews (not much different than the civilian 8 hour allowing factory work to be split into 3 shifts). How they rationalize it after the fact is a different story.