Savasana - Corpse Pose - Final Rest"The body is mortal. It is subject to death. Yet it is the resting place of the immortal, incorporeal Self." - Chandogya Upanishad
Lying prone on the floor is a traditional form of corpse pose, Savasana. It is such a common, innate, impulse to lie down this way after exertion, that it might not seem like a pose. The trick is to stay still. Yes, as still as death, knowing you will awaken.
The decreased demand on all physical systems of the body, while in this pose, facilitates accurate and efficient encoding of neuromuscular information registered during activity. In rest, the brain and body build and enhance neural networks recording the new length of muscles, position of connective tissue like fascia, and capacity of lungs, that occurred during activity just before. Hence yoga practices end with a final rest, crucial to nonconscious recall and learning - developing body clairvoyance for performance.
Lie flat on your back, or on your stomach with head turned to the side, on a floor, a bed, a couch or whatever is comfortable and check out this "sleep playlist" from 8tracks.
For corpse pose variations please visit Yoga Art and Science.
For a superior visualization for relaxing see Yogi Source's Ananda Savasana.