I have noticed a considerable dropoff in my energy levels in the afternoon. I am a much more diligent worker in the morning, full of vim and vigor, and consider myself to be very productive. The afternoon makes me sluggish. There is nothing profound there, but I wanted to mention it regardless.
If I were every to write a groundbreaking book, or produce a profound work of art, it would definitely be in the morning. There is such a sense of possibility in the morning.