For those who practice the ashtanga vinyasa yoga, that means it's time to loosen your white-knuckle grip from your yoga mat and unclench your butt cheeks to take the day off. Remember, the sphincter is ashwini mudra, not mula bandha. Anyways, the practice isn't going anywhere.
My favorite Chuck Miller story of late: Sri Miller mentioned to a friend that 2-hour practices used to be maintenance practices. Four hours per day was the norm. As Sri Freeman said in the execrably titled Yogi Bare, they used to pray for days off.