Yoga practitioners advise the times around sunrise and sunset, well before eating a meal, as the best time for yoga and meditation practice. The science of biometeorology (the study of natural forces on human and animal life) tells us that the sun has a tremendous impact upon the lives of plants, animals, and human beings. Even our blood chemistry changes with the rising and setting of the sun! Therefore, there may be a chemical basis for the thousands of years of belief, in every spiritual tradition, that to meditate and pray at sunrise and sunset is somehow more effective, more auspicious.
Vimala Schneider McClure, in A Woman's Guide to Tantra Yoga, p. 106.