When Did the "East" Become the "North"

Brad DeLong blogs the American Revolution.  Today 
he quotes Washington's letter to the Continental Congress, his second in two days, on the dire situation of his army, now camping for the winter at Valley Forge:

We have, by a field return this day made no less than 2898 Men now in Camp unfit for duty because they are bare foot and otherwise naked and by the same return it appears that our whole strength in continental Troops (Including the Eastern Brigades which have joined us since the surrender of Genl. Burgoyne)....

You should read the whole thing if you're interested in history, but what catches my interest is the reference to the "Eastern Brigades".  The context makes it apparent that he's referring to men from NY and New England, people whom we today would call "Northerners" or "Yankess".  A glance at the map shows why the reference: New England does lie to the (north)east of the mid-Atlantic states.  (Maine is still known as "downeast".)

So why and when did the "North" become the "North", rather than the "East"?  I suppose the change in terminology would be associated with the rise of sections in the new nation, perhaps even accentuating sectional tensions.