An earlier incarnation of the idea was published by Dorothea Brande in her 1934 book "Becoming a Writer" - yes, 1934! Here she advises getting up early every day and, before doing anything else, just write.
In a blog post from 2012 Ruth Livingstone laments:
Sadly, not everybody acknowledges that Dorothea was the first advocate of this technique – which she called ‘early morning writing’. And, worse still, few people seem to know exactly what Dorothea said about early morning writing, nor demonstrate much understanding of the purpose of this writing as she saw it
Case in point.
So I did a search:
Write anything that comes into your head: last night’s dream, if you are able to remember it; the activities of the day before; a conversation, real or imaginary; an examination of conscience. Write any sort of early morning reverie, rapidly and uncritically.
We're on familiar territory.
The best way to do this is to rise half an hour, or a full hour, earlier than you customarily rise. Just as soon as you can—and without talking, without reading the morning’s paper, without picking up the book you laid aside the night before—begin to write.
While the traditional approach of Morning Pages is for them to be essentially disposable, Dorothea obviously takes the writer's approach:
The excellence or ultimate worth of what you write is of no importance yet. As a matter of fact, you will find more value in this material than you expect, but your primary purpose now is not to bring forth deathless words, but to write any words at all which are not pure nonsense.
The value derived from Brande's early morning writing differs from what has become the more esoteric modern purpose:
what you are actually doing is training yourself, in the twilight zone between sleep and the full waking state, simply to write
and that the process will become easier allowing you to write more (double your efforts) until:
You will have begun to feel that you can get as much (far more really) from a written reverie as from one that goes on almost wordlessly in the back of your mind.
What is written "will have uses you can hardly foresee."