" The interpenetration of man’s three bodies is expressed in many ways through his threefold nature,” my great guru went on. “In the wakeful state on earth a human being is conscious more or less of his three vehicles. When he is sensuously intent on tasting, smelling, touching, listening, or seeing, he is working principally through his physical body. Visualizing or willing, he is working mainly through his astral body. His causal being finds expression when man is thinking or diving deep in introspection or meditation; the cosmical thoughts of genius come to the man who habitually contacts his causal body. In this sense an individual may be classified broadly as ‘a material man,’ ‘an energetic man,’ or ‘an intellectual man.’ “A man identifies himself about sixteen hours daily with his physical vehicle. Then he sleeps; if he dreams, he remains in his astral body, effortlessly creating any object even as do the astral beings. If man’s sleep be deep and dreamless, for several hours he is able to transfer his consciousness, or sense of I-ness, to the causal body; such sleep is revivifying. A dreamer is contacting his astral and not his causal body; his sleep is not fully refreshing. "

― Paramahansa Yogananda , Autobiography of a Yogi