if you don't breathe through your nose you won't achieve relaxation... and even though i was doing ashtanga (which i'll likely go back to) and it was strenuous, the fact that i could maintain the breathing through my nose kept me relaxed. if you stop breathing through your nose you should stop until you can continue.
incidentally i read in yoga magazine about a yogi who went through a major surgery w/o anesthetic by utilising yoga techniques.