This is an interesting concept and probably doesn’t mean what you think it does. In short, take care of yourself.
Knowing yourself is about knowing your health and capabilities.
I cannot go to sleep at 2 a.m. and get up four hours later at 6 a.m. to deliver a speech at 8. I’ll be exhausted, and even if my audience doesn’t notice, I will. I’ll be tired. It will be harder to concentrate.
Know yourself. Know your limits.
I have a goal when I have a speaking engagement the next morning – “In bed by 10.” It doesn’t matter how much fun I’m having the night before the speech. It doesn’t matter if the client is taking me out for a fancy dinner. It’s in bed by 10. You’ll never be at your best if you don’t do what you know is best for yourself.