"Where and how does the practice of yoga begin? Should we always begin on the physical level? I would say that where we begin depends on our personal interests. There are many ways of practicing yoga, and gradually the interest in one path will lead to another. So it could be that we begin by studying the Yoga Sutra or by meditating. Or we may instead begin with practicing asanas and so start to understand yoga through the experience of the body. We can also begin with pranayama, feeling the breath as the movement of our inner being. There are no prescriptions regarding where and how our practice should begin."
quotation from The Heart of Yoga by T.K.V. Desikachar

I think it was Patanjali who talked about a stepping up that occurs between the performing of asanas and pranayama? Is he talking about one being able to handle the increase of light in the body brought on by pranayama?

Conscious, correct breathing is encouraged by fitness experts, and today’s yoga teachers continue to teach pranayamic, breathing exercises which were created thousands of years ago. Modern world yogis understand that as increased quantities of oxygen are drawn into the body, increased quantities of toxins and waste products are removed from the cells, giving a person more vitality and energy. As well, air carries prana which is the lifeforce. So the more lifeforce in the bloodstream, the more well the body and the more well the body, the more divine light it is able to contain. Conscious breathing prepares the practitioner to become a vessel for containing more divine light than to which it was previously accustomed. Light too can burn away toxins from the mind and body. I would be interested in learning about other people's understanding of the connection between pranayama and light.