In this deep state of meditation which assumes for the outer observer the signs of trance, or half-trance, there will be some transitional moments when consciousness itself disappears, when the deepened bliss of the experience is broken by utter insensibility, when its growing light is met by darkness and when the meditator's own awareness of any kind of being at all lapses. If his moral and intellectual preparations have been sufficiently and properly made, he need have no fear of this temporary state, which will be quite brief in any event. The Indians call it "Yoga sleep," and indeed, it is as pleasant and as harmless as ordinary sleep.
Before the higher functions of the human entity's psychological machinery can displace the lower ones, it seems that Nature requires in most cases this interruption in existence, this discontinuity of awareness to take place for a few moments.
-- Notebooks Category 4: Elementary Meditation > Chapter 1 : Preparatory > # 290
