This is a perfect thread because it is essential and impossible. It touches on a whole bunch of essential elements in D/s (et al.) relationships. At the same time each relationship is different, with different agreements and different assumptions, and so generalizations are tricky. I would say that "I'm tired" is not the basis of a decision, but rather the beginning of a conversation. Maybe "I'm tired" leads to a discussion of how and why. Maybe sub really wants to be pushed, or maybe sub who has never lied to Master is sharing valuable information, so that Master can respond appropriately. What if Master is equally tired for all the reasons that we all understand, and an agreement is forged, early to bed, early to rise, with appropriate punishment in the morning for being cheeky. Ah, now there is a thought. Ever the optimist here.