Too tired to think. Too tired to eat. Too tired to do anything.
Alas, too tired is not a valid excuse. Still gotta think, still gotta eat, and still gotta do things. So if you are going to have to do things anyway, do your best with what you have. Too often people look for a perfect answer or perfect solution. Often doing just your best is all you need.
It's your best, and that is always enough. Even when it doesn't feel like it.