The most important reason for your “no” is that you need your downtime so you won’t behave like a jerk because you’re depleted. And you don’t want to battle an appetite spiked by the stress of overcommitment. But that’s your secret; others don’t need that information. So just smile, say no, thank you, and keep moving.
We have a right to decide how we want our bodies to look and feel, but unfortunately we do not exercise these rights. Instead, we tend to drift along, victims of our own ignorance of the fact that we can have what we want, if we are willing to take that first step toward developing the self-discipline to govern our thoughts.