When the practiser is really proficient he may encounter a very profound state of "yoga sleep." This is difficult to describe. It is mysterious and enigmatic. He will not even be quite sure whether or not it happened, but will probably deduce its factuality from the length of time that must have been spent in it. He will not remember anything about it since he is very vaguely aware that total unconsciousness did not occur, that it was not ordinary dreamless sleep, that some kind of spiritual experience was present of which he can form no conception and obtain no understanding. The end result after emerging will be satisfying and pleasant, calming and detaching.
-- Notebooks Category 23: Advanced Contemplation > Chapter 7: Contemplative Stillness > # 127