If I'm struggling with a run, immediately afterwards my head feels like it wants to explode out of my skull. It's feels like there's a lot of pressure in there, and it hurts! I never looked into why that happens, and it goes away after a while.
I simply cannot exercise during the morning. I have always been a night person, and mornings for me are a groggy mess. I also find that once I've eaten a few meals, I have way more energy to get down to it.