I would wonder what the sub is "too tired" for? D/s relationships and activities run an almost infinite continuum of activities. Some emotionally straining, others physically demanding, and still others an interesting extreme of both. If a sub is too tired for physically demanding play they may still be open to and possibly respond quite well to mind play or some aspect of humiliation or tasking, if those are existing elements of the relationship. IMHO, in a genuine relationship both parties should strive not only to understand and respect what "too tired" rules out, but also to explore enthusiastically what it leaves open for exploration.