8am is too early for me too!

I even postponed taking the exam a month later because they only had 8am available for the month I wanted to test. I settled on 10:30am in May.
You have time to train your body and brain to work for you...

Take practice tests at the same time your actual GMAT is going to be. Eat meals and snacks at the same intervals you would on test day. When I first took the GMAT, I had the same concerns as you! My appointment was at 2pm, right when my body starts to crash after lunch. About two weeks before the test, I started eating small snacks for lunch, instead of my usual meal. It helped me avoid the crash! However, on test day, I got so nervous, I barely got a bite down at breakfast and didn't end up eating any snacks during the test breaks! I wasn't tired though!
Good luck with your exam!