You should take advantage of these states of cognitive exhaustion by asking Claude to document and explain the codebase to you, and checking whether it still makes sense. If there are things that you have trouble understanding in that state, make a note of them to check later whether they can be simplified.
Or, you know, rest :-)
No, it's much better for the business magnates if you spend all your short lifespan on solving problems every waking hour. Don't worry about experiencing your life, you need to generate value!