The principle of capacity applies to other areas of life as well:
You can only serve someone else if you aren't overextended.
You can only demonstrate love to someone else if you aren't emotionally drained.
You can only help comfort someone who is overwhelmed if you aren't overwhelmed.
The true ability of a person might be better measured not by the time energy and emotion they expend, but by how much time, energy, and emotion they have left after they accomplish their assigned duties. The person that has something left in the tank has the opportunity to truly impact other peopled who are over capacity.
What I'm suggesting is somewhat heretical because there is an unspoken assumption that if you have extra time energy and emotion left over, you are lazy and should be doing something else. That's typically how I feel. In the end it all depends on how you use your extra capacity. If it's spent in front of the TV or at the bar by yourself, then you probably should be doing more. If you take the opportunity to help and impact someone else, I think that's time well spent.
Are you over capacity?