I've mentioned before that I've spent a lot of time training with a tai chi master who is also a doctor of traditional Chinese medicine. Yin and Yang are the two primal forces that make up the phenomenological world (the world as we experience it). Mostly the western understanding of these two forces is pretty superficial. You've probably heard about yin and yang as feminine and masculine; earth and sun or heaven; dark and light; cold and hot. They flow into each other and become each other, too. In eastern systems, they aren't "yin and yang" - they are "yin yang," two sides of the same coin, but interpenetrating.
When we sleep, we use yin energy to cleanse our bodies.
When you meditate, you can draw on yin or yang energies, depending on how you do it. To grossly simplify things, if you meditate with eyes open in a room with a light source, you will draw yang energy. Eyes closed meditations are yin.
Deng Ming-Dao's book Chronicles of Tao offers a narrative account of his master Kwan Saihung's experiences training in the Huashan mountains of China. When Kwan Saihung is still a boy, his teacher takes him to visit an immortal, a Taoist practitioner who has achieved mastery in the practice of his choice. As a sometime fan of vampire fiction, I found this account fascinating. I hope you do too. (From Chronicles of Tao, pages 70-71.)
Master and disciple walked for over an hour until they came to a tiny stucco cottage. It was plain, with only a few square windows and an old tile roof. It was summer, and every building on Huashan had its windows propped open to admit the warm sunlight. The windows of this one, however, were tightly shut. The door was slightly ajar and, after knocking, the two stepped inside.
The small interior was dark and quiet, and a flow of cool air blew on them as they entered. Still blind from the bright sun, Saihung's vision adjusted slowly. Set among a few modest furnishings, was a large coffin.
Saihung saw his master drop down to his hands and knees, and Saihung automatically followed. He was puzzled. He had only seen his master bow during ceremonies. But there was no altar here, and his master couldn't be bowing to the coffin. Saihung completed his bow and looked up. There was a tall figure standing before them.
The figure remained standing and acknowledged their bow with a slight nod.
"Hey, you!" cried Saihung, "Why don't you bow too? Don't you know how important my master is?"
"Saihung!" said the Grand Master sharply. "Don't be rude. He is the master here, not I." He turned to the man. "Greetings to the Bat Immortal."
The Bat Immortal smiled slightly. He was tall, thin, and moved in an almost feminine fashion. His face was small, his beard and hair braided with ribbons, his skin unwrinkled, pale, and bloodless. Narrow eyes were sunken, the skin around them blackened, and they were almost closed all the time. But from the narrow slits of his eyes there seemed to shine an inner light, a hidden glow.
"I've come to ask a point about the scriptures," said the Grand Master.
The Bat Immortal acknowledged the request by stepping forward. He avoided the sunlight coming through the door, and his steps were soundless. He stopped in front of Saihung. His eyelids lifted slightly; the glow from his eyes intensified.
"Is this the boy you mentioned?" he asked in a thin and hollow voice.
"Yes," replied the Grand Master.
The Bat Immortal turned back to Saihung. Saihung looked up, and he had the uncanny feeling that the Bat Immortal gazed directly through his eyelids. Saihung's attention lapsed, and when he again became aware, the Bat Immortal had turned away.
The Grand Master sent Saihung outside to wait.
When he emerged an hour later, the Grand Master walked directly away. Saihung followed him. After a half hour of silence, the Grand Master told him about the Bat Immortal.
"The Bat Immortal practices extreme yin training. That's why he has taken the name he has and sleeps in a coffin, avoids sunlight, stays only in cold places, and never eats anything hot. He cultivates the Great Yin, and this is the source of his spirituality."
"He seems like a wicked man, with those dark circles and ghostly movements," said Saihung.
"Don't think he is evil," cautioned his master. "He frightens you because he is an unfamiliar person. Naturally. He is immortal, and immortals are rarely glimpsed."
"But, Gong-Gong, I don't understand why you bow to him. Everyone always bows to you."
"Saihung, there are always greater and greater masters, and we must always show our respect."