I've also read some yoga books. Patanjali, the father of yoga, says a Yoga Asana (pose) should be a "steadfast, relaxed position" . . . in other words, it shouldn't be so hard that you feel like you're being tortured, or you won't ever be able to relax. In fact, the word "asana" translates as "seat," meaning that if you're doing a pose right, you should literally be able to "sit" in it and meditate. The purpose of yoga, according to Patanjali's Yoga Sutras, is to "calm the fluctuations of the mind." There's nothing in the Sutras about passing out from heat exhaustion.