In Chinese medicine it is believed that lying on the right side, with the knees slightly bent, the left arm and hand on the left thigh, and the right arm bent with the hand by the head, is the correct physiological position for sleep. There are two simple reasons for this. The first is that this position allows pressure to be removed from the heart, thereby allowing more efficient circulation while the body is not in motion. The second is that this position allows blood to more easily fill the tissue of the liver (on the right side of the body), which swells during sleep in order to decrease the volume of blood in the peripheral circulatory system. Chinese medicine says that a sufficient supply of Liver Blood roots the Hun to allow for sound sleep. This reflects the idea of being grounded, or rooted. This position for sleep also reflects the Chinese concept that the left side of the body is more Yang, or active, while the right is more Yin, or passive. Allowing more blood to fill the right side of the body supplements the relatively more Yin, passive act of sleeping.