What time of the day do you think you're at your peak, or your mind is clearest?
For me it's the evening, usually after 8 pm. Having taken three AP classes last year my body's all screwed up now, and having had to spend so many nights studying until 2 am has made that time of the day the best for me to think and get stuff done.
I'm usually up until about 1 am now, but that's mostly because that's when my mind finally starts to wind down; it's not too bad most of the time, but a pain when you have a 9 am class. lol