IF you're doing work that is meaningful and worth it you will be tired. Don't wait until you're not. Embrace it as part of the work. Embrace doing it early. Put it into the hands of someone who cares about it EARLY; don't wait until you're fully done or have figured it out. Do the hard part early and then EARLY put it into the hands of those who care so they help you pull it forward, feedback early before you waste time and others effort and attention on things you will cut once you understand the constraints better.