Browsing an old book, H. E. Wingfield's "An Introduction to the Study of Hypnotism" (London, 1920, pages 26-27), I was surprised to read the paragraph below, mentioning the idea of a continuous spectrum of consciousness ranging from waking to deep somnambulism and beyond, in 1920, well before Monroe, then Kepple mentioned it. I wouldn't be surprised to learn that this idea is even much older.
"On the whole, perhaps, one may regard the range of consciousness as analogous to the solar spectrum. The waking consciousness might be represented by the red end of the spectrum, the profound somnambulistic consciousness by the violet, and the deepest part, which, we may suppose, is in relation with the visceral functions, by the ultra-violet invisible rays. Clearly, if we examine the red end of the spectrum, and compare it with the violet, we shall find a sharp line of demarcation between the two, for we shall have missed out the orange which bridges them. But if we look at the whole spectrum, it is obvious that it is continuous from end to end."
That's a great find and I agree that Monroe and Kepple certainly were not the first. I could have written many of the books on the subject myself. Thing is I didn't, they did and regardless who wrote them it is a opportunity for exploration. That last line of the quote really sums it up nicely.