Traditional yoga practice is timed according to what are called "junctions" (sandhya) of the day. ("Junction," by the way, has the same linguistic root as yoga.)
Two of these junctions are especially important: one at dawn, the other at twilight. Just as there are energetic "power spots" on the Earth, there are "power times" during the day. To understand what this means, let's recall the two definitions, literal and symbolic, of the Sanskrit word hatha. Literally, hatha means "force," and so hatha yoga strictly means the "forceful union" method. But when defined symbolically, the word is divided into its constituent syllables, ha-tha, with the first interpreted as "sun," and the second as "moon."
This was once a highly esoteric teaching. Just as a sun and a moon exist in the exterior world, there are a Sun and Moon in our interior subtle world as well. This concept is based on the ancient principle (also known to the West) that the human body is a miniature representation of the universe. We can read in the ancient yoga text, the Shiva Samhita: "In this body are...rivers, oceans, realms, and rulers. There are seers, sages, all the constellations and planets...the moon and sun...are revolving."
If we delve deeper, we learn that Sun and Moon stand for the "heating" (or "lightening") and "cooling" (or "darkening") energies that, like an alternating electrical current, power our lives. For the average person, these energies are constantly shifting their dominance throughout the day, buffeting the poor nonyogi back and forth and creating all sorts of discomfort. The yogi, however, trains herself to balance and conserve these energies, never to be too "light" or too "dark." What better time to practice than either dawn or twilight, when all around you light and dark are in harmony? When your schedule permits, try it yourself. You just may find that the equilibrium of the outer world has a marked effect on your practice.