Falling into stillness through conscious meditation before sleep is known as "nourishing the spirit and body," also called "embryonic breathing."
This was the most optimal resting pattern. With consciousness entering stillness, hovering somewhere between a dream and reality, one's body instincts and finely tuned awareness would consume stored nutrients within to repair and shape the physique in a flawless manner.
Just one or two hours of such spiritual sleep could satisfy a normal person's daily needs, and even physical depletion from injuries could be mended by sleeping for a bit longer.
Chapter 55