I. Mythology behind yoga nidra

In the Patanjali Charitham, the legend of sage Patanjali, composed by Ramabhadra Dixit in the 17th century, the story starts with Lord Vishnu reposing in yoga nidra or cosmic sleep where he meditates on the universe. As it turns out, Lord Vishnu tends to get heavier during yoga nidra. This particular time, Lord Vishnu having entered such a deep state of yoga nidra, became extremely heavy, so heavy that Adisesha, the thousand headed serpent on whom Vishnu is sleeping, could no longer bear his weight and as a result was huffing and puffing. When a thousand headed serpent huffs and puffs, it results in venom being ejected. The toxic fumes caused by Adisesha's huffs and puffs freightened the devas (the divines) who prayed to Vishnu to wake up from his yoga nidra. Vishnu eventually did wake up and became lighter after he did. Adisesha could breathe normally again and as a result the world was saved.

3) A sense of heaviness in yoganidrasana

Many Asanas, if done correctly, will result in a sense of lightness due to the upward flow of prana. The Hatha Yoga Pradipika calls this Anga Laghavam (lightness of the various parts of the body). Asanas like Tittibasana (shown to the right) and Dwipada Shirashasana (below) especially induce a sense of lightness. Yoga Nidrasana on the other hand induces a sense of heaviness. What is the reason for this ?

The answer lies in the spine. In Dwipada Shirashasana, the prone version of the supine yoganidrasana, we sit on the upper part of the buttock and make an effort to keep the spine straight. This induces a sense of alertness. In yoganidrasana, there is no effort to elongate the spine. The only effort needed is to tilt the head back and rest it on the feet.