How do I know if I'm too exhausted from work?

You run on adrenaline and too little sleep, and somewhere along the way you started calling that normal. Here is your answer: if you had to ask, you already know. Exhaustion becomes dangerous exactly when you stop noticing it, when a level of tired that should alarm you just feels like Tuesday. The body does not lie, even when the mind talks itself into being fine. Constant fatigue, a short fuse, getting sick often, forgetting things, not caring about things you used to. Those are not signs of toughness. They are signals that you are borrowing from tomorrow to get through today, and that debt comes due. So stop measuring yourself against how much you can endure. Measure against how a rested person actually feels. Pick one night this week and protect your sleep like it is an appointment you cannot miss. Notice how different you feel the next day. Let that difference be your proof that what you have been accepting is not okay, and does not have to be.